Retire completed diff plans for release preparation

AGENTS.md

@@ -1,5 +1,25 @@
-Please review `docs/development.md` for initial orientation in this repository
-and follow its task-specific reading guide.
+<!-- codebase-memory-mcp:start -->
+# Codebase Knowledge Graph (codebase-memory-mcp)
 
-Review and follow `docs/policy/agents.md` before recommending, planning, or
-implementing any changes to this repository.
+This project uses codebase-memory-mcp to maintain a knowledge graph of the codebase.
+ALWAYS prefer MCP graph tools over grep/glob/file-search for code discovery.
+
+## Priority Order
+1. `search_graph` — find functions, classes, routes, variables by pattern
+2. `trace_path` — trace who calls a function or what it calls
+3. `get_code_snippet` — read specific function/class source code
+4. `query_graph` — run Cypher queries for complex patterns
+5. `get_architecture` — high-level project summary
+
+## When to fall back to grep/glob
+- Searching for string literals, error messages, config values
+- Searching non-code files (Dockerfiles, shell scripts, configs)
+- When MCP tools return insufficient results
+<!-- codebase-memory-mcp:end -->
+
+Start with `docs/development.md` for orientation and use its policy reading guide
+to select the policies applicable to the task. Do not read the entire suite by
+default.
+
+Read and follow `docs/policy/agents.md` before recommending, planning, or
+implementing repository changes.

COPYRIGHT

@@ -0,0 +1,25 @@
+ConfigKit
+
+Copyright (C) 2026 Eric Rakestraw
+
+SPDX-License-Identifier: LGPL-3.0-or-later
+
+This library is free software: you can redistribute it and/or modify it
+under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+The GNU Lesser General Public License version 3 and its incorporated
+GNU General Public License version 3 text are provided in LICENSE.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU Lesser General Public License for more details.
+
+Third-party code and assets are subject to their respective license terms.
+
+Policy documents adapted from Standard Repository Policies, the root AGENTS.md
+orientation, and the Policy Reading Guide in docs/development.md remain licensed
+under CC-BY-SA-4.0. See docs/oss/attribution.md for the affected material,
+upstream credit, and license links.

LICENSE

@@ -1,304 +1,840 @@
                    GNU LESSER GENERAL PUBLIC LICENSE
                        Version 3, 29 June 2007
 
-Copyright (C) 2007 Free Software Foundation, Inc. <http://fsf.org/>
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
 
-Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
 
-This version of the GNU Lesser General Public License incorporates the terms and conditions of version 3 of the GNU General Public License, supplemented by the additional permissions listed below.
+  This version of the GNU Lesser General Public License incorporates
+the terms and conditions of version 3 of the GNU General Public
+License, supplemented by the additional permissions listed below.
 
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-As used herein, "this License" refers to version 3 of the GNU Lesser General Public License, and the "GNU GPL" refers to version 3 of the GNU General Public License.
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   5. Combined Libraries.
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                     GNU GENERAL PUBLIC LICENSE
                        Version 3, 29 June 2007
 
-Copyright © 2007 Free Software Foundation, Inc. <http://fsf.org/>
-
-Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
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+
+  Conveying under any other circumstances is permitted solely under
+the conditions stated below.  Sublicensing is not allowed; section 10
+makes it unnecessary.
 
   3. Protecting Users' Legal Rights From Anti-Circumvention Law.
-No covered work shall be deemed part of an effective technological measure under any applicable law fulfilling obligations under article 11 of the WIPO copyright treaty adopted on 20 December 1996, or similar laws prohibiting or restricting circumvention of such measures.
 
-When you convey a covered work, you waive any legal power to forbid circumvention of technological measures to the extent such circumvention is effected by exercising rights under this License with respect to the covered work, and you disclaim any intention to limit operation or modification of the work as a means of enforcing, against the work's users, your or third parties' legal rights to forbid circumvention of technological measures.
+  No covered work shall be deemed part of an effective technological
+measure under any applicable law fulfilling obligations under article
+11 of the WIPO copyright treaty adopted on 20 December 1996, or
+similar laws prohibiting or restricting circumvention of such
+measures.
+
+  When you convey a covered work, you waive any legal power to forbid
+circumvention of technological measures to the extent such circumvention
+is effected by exercising rights under this License with respect to
+the covered work, and you disclaim any intention to limit operation or
+modification of the work as a means of enforcing, against the work's
+users, your or third parties' legal rights to forbid circumvention of
+technological measures.
 
   4. Conveying Verbatim Copies.
-You may convey verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice; keep intact all notices stating that this License and any non-permissive terms added in accord with section 7 apply to the code; keep intact all notices of the absence of any warranty; and give all recipients a copy of this License along with the Program.
 
-You may charge any price or no price for each copy that you convey, and you may offer support or warranty protection for a fee.
+  You may convey verbatim copies of the Program's source code as you
+receive it, in any medium, provided that you conspicuously and
+appropriately publish on each copy an appropriate copyright notice;
+keep intact all notices stating that this License and any
+non-permissive terms added in accord with section 7 apply to the code;
+keep intact all notices of the absence of any warranty; and give all
+recipients a copy of this License along with the Program.
+
+  You may charge any price or no price for each copy that you convey,
+and you may offer support or warranty protection for a fee.
 
   5. Conveying Modified Source Versions.
-You may convey a work based on the Program, or the modifications to produce it from the Program, in the form of source code under the terms of section 4, provided that you also meet all of these conditions:
 
-     a) The work must carry prominent notices stating that you modified it, and giving a relevant date.
+  You may convey a work based on the Program, or the modifications to
+produce it from the Program, in the form of source code under the
+terms of section 4, provided that you also meet all of these conditions:
 
-     b) The work must carry prominent notices stating that it is released under this License and any conditions added under section 7. This requirement modifies the requirement in section 4 to “keep intact all notices”.
+    a) The work must carry prominent notices stating that you modified
+    it, and giving a relevant date.
 
-     c) You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy. This License will therefore apply, along with any applicable section 7 additional terms, to the whole of the work, and all its parts, regardless of how they are packaged. This License gives no permission to license the work in any other way, but it does not invalidate such permission if you have separately received it.
+    b) The work must carry prominent notices stating that it is
+    released under this License and any conditions added under section
+    7.  This requirement modifies the requirement in section 4 to
+    "keep intact all notices".
 
-     d) If the work has interactive user interfaces, each must display Appropriate Legal Notices; however, if the Program has interactive interfaces that do not display Appropriate Legal Notices, your work need not make them do so.
+    c) You must license the entire work, as a whole, under this
+    License to anyone who comes into possession of a copy.  This
+    License will therefore apply, along with any applicable section 7
+    additional terms, to the whole of the work, and all its parts,
+    regardless of how they are packaged.  This License gives no
+    permission to license the work in any other way, but it does not
+    invalidate such permission if you have separately received it.
 
-A compilation of a covered work with other separate and independent works, which are not by their nature extensions of the covered work, and which are not combined with it such as to form a larger program, in or on a volume of a storage or distribution medium, is called an “aggregate” if the compilation and its resulting copyright are not used to limit the access or legal rights of the compilation's users beyond what the individual works permit. Inclusion of a covered work in an aggregate does not cause this License to apply to the other parts of the aggregate.
+    d) If the work has interactive user interfaces, each must display
+    Appropriate Legal Notices; however, if the Program has interactive
+    interfaces that do not display Appropriate Legal Notices, your
+    work need not make them do so.
+
+  A compilation of a covered work with other separate and independent
+works, which are not by their nature extensions of the covered work,
+and which are not combined with it such as to form a larger program,
+in or on a volume of a storage or distribution medium, is called an
+"aggregate" if the compilation and its resulting copyright are not
+used to limit the access or legal rights of the compilation's users
+beyond what the individual works permit.  Inclusion of a covered work
+in an aggregate does not cause this License to apply to the other
+parts of the aggregate.
 
   6. Conveying Non-Source Forms.
-You may convey a covered work in object code form under the terms of sections 4 and 5, provided that you also convey the machine-readable Corresponding Source under the terms of this License, in one of these ways:
 
-     a) Convey the object code in, or embodied in, a physical product (including a physical distribution medium), accompanied by the Corresponding Source fixed on a durable physical medium customarily used for software interchange.
+  You may convey a covered work in object code form under the terms
+of sections 4 and 5, provided that you also convey the
+machine-readable Corresponding Source under the terms of this License,
+in one of these ways:
 
-     b) Convey the object code in, or embodied in, a physical product (including a physical distribution medium), accompanied by a written offer, valid for at least three years and valid for as long as you offer spare parts or customer support for that product model, to give anyone who possesses the object code either (1) a copy of the Corresponding Source for all the software in the product that is covered by this License, on a durable physical medium customarily used for software interchange, for a price no more than your reasonable cost of physically performing this conveying of source, or (2) access to copy the Corresponding Source from a network server at no charge.
+    a) Convey the object code in, or embodied in, a physical product
+    (including a physical distribution medium), accompanied by the
+    Corresponding Source fixed on a durable physical medium
+    customarily used for software interchange.
 
-     c) Convey individual copies of the object code with a copy of the written offer to provide the Corresponding Source. This alternative is allowed only occasionally and noncommercially, and only if you received the object code with such an offer, in accord with subsection 6b.
+    b) Convey the object code in, or embodied in, a physical product
+    (including a physical distribution medium), accompanied by a
+    written offer, valid for at least three years and valid for as
+    long as you offer spare parts or customer support for that product
+    model, to give anyone who possesses the object code either (1) a
+    copy of the Corresponding Source for all the software in the
+    product that is covered by this License, on a durable physical
+    medium customarily used for software interchange, for a price no
+    more than your reasonable cost of physically performing this
+    conveying of source, or (2) access to copy the
+    Corresponding Source from a network server at no charge.
 
-     d) Convey the object code by offering access from a designated place (gratis or for a charge), and offer equivalent access to the Corresponding Source in the same way through the same place at no further charge. You need not require recipients to copy the Corresponding Source along with the object code. If the place to copy the object code is a network server, the Corresponding Source may be on a different server (operated by you or a third party) that supports equivalent copying facilities, provided you maintain clear directions next to the object code saying where to find the Corresponding Source. Regardless of what server hosts the Corresponding Source, you remain obligated to ensure that it is available for as long as needed to satisfy these requirements.
+    c) Convey individual copies of the object code with a copy of the
+    written offer to provide the Corresponding Source.  This
+    alternative is allowed only occasionally and noncommercially, and
+    only if you received the object code with such an offer, in accord
+    with subsection 6b.
 
-     e) Convey the object code using peer-to-peer transmission, provided you inform other peers where the object code and Corresponding Source of the work are being offered to the general public at no charge under subsection 6d.
+    d) Convey the object code by offering access from a designated
+    place (gratis or for a charge), and offer equivalent access to the
+    Corresponding Source in the same way through the same place at no
+    further charge.  You need not require recipients to copy the
+    Corresponding Source along with the object code.  If the place to
+    copy the object code is a network server, the Corresponding Source
+    may be on a different server (operated by you or a third party)
+    that supports equivalent copying facilities, provided you maintain
+    clear directions next to the object code saying where to find the
+    Corresponding Source.  Regardless of what server hosts the
+    Corresponding Source, you remain obligated to ensure that it is
+    available for as long as needed to satisfy these requirements.
 
-A separable portion of the object code, whose source code is excluded from the Corresponding Source as a System Library, need not be included in conveying the object code work.
+    e) Convey the object code using peer-to-peer transmission, provided
+    you inform other peers where the object code and Corresponding
+    Source of the work are being offered to the general public at no
+    charge under subsection 6d.
 
-A “User Product” is either (1) a “consumer product”, which means any tangible personal property which is normally used for personal, family, or household purposes, or (2) anything designed or sold for incorporation into a dwelling. In determining whether a product is a consumer product, doubtful cases shall be resolved in favor of coverage. For a particular product received by a particular user, “normally used” refers to a typical or common use of that class of product, regardless of the status of the particular user or of the way in which the particular user actually uses, or expects or is expected to use, the product. A product is a consumer product regardless of whether the product has substantial commercial, industrial or non-consumer uses, unless such uses represent the only significant mode of use of the product.
+  A separable portion of the object code, whose source code is excluded
+from the Corresponding Source as a System Library, need not be
+included in conveying the object code work.
 
-“Installation Information” for a User Product means any methods, procedures, authorization keys, or other information required to install and execute modified versions of a covered work in that User Product from a modified version of its Corresponding Source. The information must suffice to ensure that the continued functioning of the modified object code is in no case prevented or interfered with solely because modification has been made.
+  A "User Product" is either (1) a "consumer product", which means any
+tangible personal property which is normally used for personal, family,
+or household purposes, or (2) anything designed or sold for incorporation
+into a dwelling.  In determining whether a product is a consumer product,
+doubtful cases shall be resolved in favor of coverage.  For a particular
+product received by a particular user, "normally used" refers to a
+typical or common use of that class of product, regardless of the status
+of the particular user or of the way in which the particular user
+actually uses, or expects or is expected to use, the product.  A product
+is a consumer product regardless of whether the product has substantial
+commercial, industrial or non-consumer uses, unless such uses represent
+the only significant mode of use of the product.
 
-If you convey an object code work under this section in, or with, or specifically for use in, a User Product, and the conveying occurs as part of a transaction in which the right of possession and use of the User Product is transferred to the recipient in perpetuity or for a fixed term (regardless of how the transaction is characterized), the Corresponding Source conveyed under this section must be accompanied by the Installation Information. But this requirement does not apply if neither you nor any third party retains the ability to install modified object code on the User Product (for example, the work has been installed in ROM).
+  "Installation Information" for a User Product means any methods,
+procedures, authorization keys, or other information required to install
+and execute modified versions of a covered work in that User Product from
+a modified version of its Corresponding Source.  The information must
+suffice to ensure that the continued functioning of the modified object
+code is in no case prevented or interfered with solely because
+modification has been made.
 
-The requirement to provide Installation Information does not include a requirement to continue to provide support service, warranty, or updates for a work that has been modified or installed by the recipient, or for the User Product in which it has been modified or installed. Access to a network may be denied when the modification itself materially and adversely affects the operation of the network or violates the rules and protocols for communication across the network.
+  If you convey an object code work under this section in, or with, or
+specifically for use in, a User Product, and the conveying occurs as
+part of a transaction in which the right of possession and use of the
+User Product is transferred to the recipient in perpetuity or for a
+fixed term (regardless of how the transaction is characterized), the
+Corresponding Source conveyed under this section must be accompanied
+by the Installation Information.  But this requirement does not apply
+if neither you nor any third party retains the ability to install
+modified object code on the User Product (for example, the work has
+been installed in ROM).
 
-Corresponding Source conveyed, and Installation Information provided, in accord with this section must be in a format that is publicly documented (and with an implementation available to the public in source code form), and must require no special password or key for unpacking, reading or copying.
+  The requirement to provide Installation Information does not include a
+requirement to continue to provide support service, warranty, or updates
+for a work that has been modified or installed by the recipient, or for
+the User Product in which it has been modified or installed.  Access to a
+network may be denied when the modification itself materially and
+adversely affects the operation of the network or violates the rules and
+protocols for communication across the network.
+
+  Corresponding Source conveyed, and Installation Information provided,
+in accord with this section must be in a format that is publicly
+documented (and with an implementation available to the public in
+source code form), and must require no special password or key for
+unpacking, reading or copying.
 
   7. Additional Terms.
-“Additional permissions” are terms that supplement the terms of this License by making exceptions from one or more of its conditions. Additional permissions that are applicable to the entire Program shall be treated as though they were included in this License, to the extent that they are valid under applicable law. If additional permissions apply only to part of the Program, that part may be used separately under those permissions, but the entire Program remains governed by this License without regard to the additional permissions.
 
-When you convey a copy of a covered work, you may at your option remove any additional permissions from that copy, or from any part of it. (Additional permissions may be written to require their own removal in certain cases when you modify the work.) You may place additional permissions on material, added by you to a covered work, for which you have or can give appropriate copyright permission.
+  "Additional permissions" are terms that supplement the terms of this
+License by making exceptions from one or more of its conditions.
+Additional permissions that are applicable to the entire Program shall
+be treated as though they were included in this License, to the extent
+that they are valid under applicable law.  If additional permissions
+apply only to part of the Program, that part may be used separately
+under those permissions, but the entire Program remains governed by
+this License without regard to the additional permissions.
 
-Notwithstanding any other provision of this License, for material you add to a covered work, you may (if authorized by the copyright holders of that material) supplement the terms of this License with terms:
+  When you convey a copy of a covered work, you may at your option
+remove any additional permissions from that copy, or from any part of
+it.  (Additional permissions may be written to require their own
+removal in certain cases when you modify the work.)  You may place
+additional permissions on material, added by you to a covered work,
+for which you have or can give appropriate copyright permission.
 
-     a) Disclaiming warranty or limiting liability differently from the terms of sections 15 and 16 of this License; or
+  Notwithstanding any other provision of this License, for material you
+add to a covered work, you may (if authorized by the copyright holders of
+that material) supplement the terms of this License with terms:
 
-     b) Requiring preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it; or
+    a) Disclaiming warranty or limiting liability differently from the
+    terms of sections 15 and 16 of this License; or
 
-     c) Prohibiting misrepresentation of the origin of that material, or requiring that modified versions of such material be marked in reasonable ways as different from the original version; or
+    b) Requiring preservation of specified reasonable legal notices or
+    author attributions in that material or in the Appropriate Legal
+    Notices displayed by works containing it; or
 
-     d) Limiting the use for publicity purposes of names of licensors or authors of the material; or
+    c) Prohibiting misrepresentation of the origin of that material, or
+    requiring that modified versions of such material be marked in
+    reasonable ways as different from the original version; or
 
-     e) Declining to grant rights under trademark law for use of some trade names, trademarks, or service marks; or
+    d) Limiting the use for publicity purposes of names of licensors or
+    authors of the material; or
 
-     f) Requiring indemnification of licensors and authors of that material by anyone who conveys the material (or modified versions of it) with contractual assumptions of liability to the recipient, for any liability that these contractual assumptions directly impose on those licensors and authors.
+    e) Declining to grant rights under trademark law for use of some
+    trade names, trademarks, or service marks; or
 
-All other non-permissive additional terms are considered “further restrictions” within the meaning of section 10. If the Program as you received it, or any part of it, contains a notice stating that it is governed by this License along with a term that is a further restriction, you may remove that term. If a license document contains a further restriction but permits relicensing or conveying under this License, you may add to a covered work material governed by the terms of that license document, provided that the further restriction does not survive such relicensing or conveying.
+    f) Requiring indemnification of licensors and authors of that
+    material by anyone who conveys the material (or modified versions of
+    it) with contractual assumptions of liability to the recipient, for
+    any liability that these contractual assumptions directly impose on
+    those licensors and authors.
 
-If you add terms to a covered work in accord with this section, you must place, in the relevant source files, a statement of the additional terms that apply to those files, or a notice indicating where to find the applicable terms.
+  All other non-permissive additional terms are considered "further
+restrictions" within the meaning of section 10.  If the Program as you
+received it, or any part of it, contains a notice stating that it is
+governed by this License along with a term that is a further
+restriction, you may remove that term.  If a license document contains
+a further restriction but permits relicensing or conveying under this
+License, you may add to a covered work material governed by the terms
+of that license document, provided that the further restriction does
+not survive such relicensing or conveying.
 
-Additional terms, permissive or non-permissive, may be stated in the form of a separately written license, or stated as exceptions; the above requirements apply either way.
+  If you add terms to a covered work in accord with this section, you
+must place, in the relevant source files, a statement of the
+additional terms that apply to those files, or a notice indicating
+where to find the applicable terms.
+
+  Additional terms, permissive or non-permissive, may be stated in the
+form of a separately written license, or stated as exceptions;
+the above requirements apply either way.
 
   8. Termination.
-You may not propagate or modify a covered work except as expressly provided under this License. Any attempt otherwise to propagate or modify it is void, and will automatically terminate your rights under this License (including any patent licenses granted under the third paragraph of section 11).
 
-However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
+  You may not propagate or modify a covered work except as expressly
+provided under this License.  Any attempt otherwise to propagate or
+modify it is void, and will automatically terminate your rights under
+this License (including any patent licenses granted under the third
+paragraph of section 11).
 
-Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.
+  However, if you cease all violation of this License, then your
+license from a particular copyright holder is reinstated (a)
+provisionally, unless and until the copyright holder explicitly and
+finally terminates your license, and (b) permanently, if the copyright
+holder fails to notify you of the violation by some reasonable means
+prior to 60 days after the cessation.
 
-Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, you do not qualify to receive new licenses for the same material under section 10.
+  Moreover, your license from a particular copyright holder is
+reinstated permanently if the copyright holder notifies you of the
+violation by some reasonable means, this is the first time you have
+received notice of violation of this License (for any work) from that
+copyright holder, and you cure the violation prior to 30 days after
+your receipt of the notice.
+
+  Termination of your rights under this section does not terminate the
+licenses of parties who have received copies or rights from you under
+this License.  If your rights have been terminated and not permanently
+reinstated, you do not qualify to receive new licenses for the same
+material under section 10.
 
   9. Acceptance Not Required for Having Copies.
-You are not required to accept this License in order to receive or run a copy of the Program. Ancillary propagation of a covered work occurring solely as a consequence of using peer-to-peer transmission to receive a copy likewise does not require acceptance. However, nothing other than this License grants you permission to propagate or modify any covered work. These actions infringe copyright if you do not accept this License. Therefore, by modifying or propagating a covered work, you indicate your acceptance of this License to do so.
+
+  You are not required to accept this License in order to receive or
+run a copy of the Program.  Ancillary propagation of a covered work
+occurring solely as a consequence of using peer-to-peer transmission
+to receive a copy likewise does not require acceptance.  However,
+nothing other than this License grants you permission to propagate or
+modify any covered work.  These actions infringe copyright if you do
+not accept this License.  Therefore, by modifying or propagating a
+covered work, you indicate your acceptance of this License to do so.
 
   10. Automatic Licensing of Downstream Recipients.
-Each time you convey a covered work, the recipient automatically receives a license from the original licensors, to run, modify and propagate that work, subject to this License. You are not responsible for enforcing compliance by third parties with this License.
 
-An “entity transaction” is a transaction transferring control of an organization, or substantially all assets of one, or subdividing an organization, or merging organizations. If propagation of a covered work results from an entity transaction, each party to that transaction who receives a copy of the work also receives whatever licenses to the work the party's predecessor in interest had or could give under the previous paragraph, plus a right to possession of the Corresponding Source of the work from the predecessor in interest, if the predecessor has it or can get it with reasonable efforts.
+  Each time you convey a covered work, the recipient automatically
+receives a license from the original licensors, to run, modify and
+propagate that work, subject to this License.  You are not responsible
+for enforcing compliance by third parties with this License.
 
-You may not impose any further restrictions on the exercise of the rights granted or affirmed under this License. For example, you may not impose a license fee, royalty, or other charge for exercise of rights granted under this License, and you may not initiate litigation (including a cross-claim or counterclaim in a lawsuit) alleging that any patent claim is infringed by making, using, selling, offering for sale, or importing the Program or any portion of it.
+  An "entity transaction" is a transaction transferring control of an
+organization, or substantially all assets of one, or subdividing an
+organization, or merging organizations.  If propagation of a covered
+work results from an entity transaction, each party to that
+transaction who receives a copy of the work also receives whatever
+licenses to the work the party's predecessor in interest had or could
+give under the previous paragraph, plus a right to possession of the
+Corresponding Source of the work from the predecessor in interest, if
+the predecessor has it or can get it with reasonable efforts.
+
+  You may not impose any further restrictions on the exercise of the
+rights granted or affirmed under this License.  For example, you may
+not impose a license fee, royalty, or other charge for exercise of
+rights granted under this License, and you may not initiate litigation
+(including a cross-claim or counterclaim in a lawsuit) alleging that
+any patent claim is infringed by making, using, selling, offering for
+sale, or importing the Program or any portion of it.
 
   11. Patents.
-A “contributor” is a copyright holder who authorizes use under this License of the Program or a work on which the Program is based. The work thus licensed is called the contributor's “contributor version”.
 
-A contributor's “essential patent claims” are all patent claims owned or controlled by the contributor, whether already acquired or hereafter acquired, that would be infringed by some manner, permitted by this License, of making, using, or selling its contributor version, but do not include claims that would be infringed only as a consequence of further modification of the contributor version. For purposes of this definition, “control” includes the right to grant patent sublicenses in a manner consistent with the requirements of this License.
+  A "contributor" is a copyright holder who authorizes use under this
+License of the Program or a work on which the Program is based.  The
+work thus licensed is called the contributor's "contributor version".
 
-Each contributor grants you a non-exclusive, worldwide, royalty-free patent license under the contributor's essential patent claims, to make, use, sell, offer for sale, import and otherwise run, modify and propagate the contents of its contributor version.
+  A contributor's "essential patent claims" are all patent claims
+owned or controlled by the contributor, whether already acquired or
+hereafter acquired, that would be infringed by some manner, permitted
+by this License, of making, using, or selling its contributor version,
+but do not include claims that would be infringed only as a
+consequence of further modification of the contributor version.  For
+purposes of this definition, "control" includes the right to grant
+patent sublicenses in a manner consistent with the requirements of
+this License.
 
-In the following three paragraphs, a “patent license” is any express agreement or commitment, however denominated, not to enforce a patent (such as an express permission to practice a patent or covenant not to sue for patent infringement). To “grant” such a patent license to a party means to make such an agreement or commitment not to enforce a patent against the party.
+  Each contributor grants you a non-exclusive, worldwide, royalty-free
+patent license under the contributor's essential patent claims, to
+make, use, sell, offer for sale, import and otherwise run, modify and
+propagate the contents of its contributor version.
 
-If you convey a covered work, knowingly relying on a patent license, and the Corresponding Source of the work is not available for anyone to copy, free of charge and under the terms of this License, through a publicly available network server or other readily accessible means, then you must either (1) cause the Corresponding Source to be so available, or (2) arrange to deprive yourself of the benefit of the patent license for this particular work, or (3) arrange, in a manner consistent with the requirements of this License, to extend the patent license to downstream recipients. “Knowingly relying” means you have actual knowledge that, but for the patent license, your conveying the covered work in a country, or your recipient's use of the covered work in a country, would infringe one or more identifiable patents in that country that you have reason to believe are valid.
+  In the following three paragraphs, a "patent license" is any express
+agreement or commitment, however denominated, not to enforce a patent
+(such as an express permission to practice a patent or covenant not to
+sue for patent infringement).  To "grant" such a patent license to a
+party means to make such an agreement or commitment not to enforce a
+patent against the party.
 
-If, pursuant to or in connection with a single transaction or arrangement, you convey, or propagate by procuring conveyance of, a covered work, and grant a patent license to some of the parties receiving the covered work authorizing them to use, propagate, modify or convey a specific copy of the covered work, then the patent license you grant is automatically extended to all recipients of the covered work and works based on it.
+  If you convey a covered work, knowingly relying on a patent license,
+and the Corresponding Source of the work is not available for anyone
+to copy, free of charge and under the terms of this License, through a
+publicly available network server or other readily accessible means,
+then you must either (1) cause the Corresponding Source to be so
+available, or (2) arrange to deprive yourself of the benefit of the
+patent license for this particular work, or (3) arrange, in a manner
+consistent with the requirements of this License, to extend the patent
+license to downstream recipients.  "Knowingly relying" means you have
+actual knowledge that, but for the patent license, your conveying the
+covered work in a country, or your recipient's use of the covered work
+in a country, would infringe one or more identifiable patents in that
+country that you have reason to believe are valid.
 
-A patent license is “discriminatory” if it does not include within the scope of its coverage, prohibits the exercise of, or is conditioned on the non-exercise of one or more of the rights that are specifically granted under this License. You may not convey a covered work if you are a party to an arrangement with a third party that is in the business of distributing software, under which you make payment to the third party based on the extent of your activity of conveying the work, and under which the third party grants, to any of the parties who would receive the covered work from you, a discriminatory patent license (a) in connection with copies of the covered work conveyed by you (or copies made from those copies), or (b) primarily for and in connection with specific products or compilations that contain the covered work, unless you entered into that arrangement, or that patent license was granted, prior to 28 March 2007.
+  If, pursuant to or in connection with a single transaction or
+arrangement, you convey, or propagate by procuring conveyance of, a
+covered work, and grant a patent license to some of the parties
+receiving the covered work authorizing them to use, propagate, modify
+or convey a specific copy of the covered work, then the patent license
+you grant is automatically extended to all recipients of the covered
+work and works based on it.
 
-Nothing in this License shall be construed as excluding or limiting any implied license or other defenses to infringement that may otherwise be available to you under applicable patent law.
+  A patent license is "discriminatory" if it does not include within
+the scope of its coverage, prohibits the exercise of, or is
+conditioned on the non-exercise of one or more of the rights that are
+specifically granted under this License.  You may not convey a covered
+work if you are a party to an arrangement with a third party that is
+in the business of distributing software, under which you make payment
+to the third party based on the extent of your activity of conveying
+the work, and under which the third party grants, to any of the
+parties who would receive the covered work from you, a discriminatory
+patent license (a) in connection with copies of the covered work
+conveyed by you (or copies made from those copies), or (b) primarily
+for and in connection with specific products or compilations that
+contain the covered work, unless you entered into that arrangement,
+or that patent license was granted, prior to 28 March 2007.
+
+  Nothing in this License shall be construed as excluding or limiting
+any implied license or other defenses to infringement that may
+otherwise be available to you under applicable patent law.
 
   12. No Surrender of Others' Freedom.
-If conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot convey a covered work so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not convey it at all. For example, if you agree to terms that obligate you to collect a royalty for further conveying from those to whom you convey the Program, the only way you could satisfy both those terms and this License would be to refrain entirely from conveying the Program.
+
+  If conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot convey a
+covered work so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you may
+not convey it at all.  For example, if you agree to terms that obligate you
+to collect a royalty for further conveying from those to whom you convey
+the Program, the only way you could satisfy both those terms and this
+License would be to refrain entirely from conveying the Program.
 
   13. Use with the GNU Affero General Public License.
-Notwithstanding any other provision of this License, you have permission to link or combine any covered work with a work licensed under version 3 of the GNU Affero General Public License into a single combined work, and to convey the resulting work. The terms of this License will continue to apply to the part which is the covered work, but the special requirements of the GNU Affero General Public License, section 13, concerning interaction through a network will apply to the combination as such.
+
+  Notwithstanding any other provision of this License, you have
+permission to link or combine any covered work with a work licensed
+under version 3 of the GNU Affero General Public License into a single
+combined work, and to convey the resulting work.  The terms of this
+License will continue to apply to the part which is the covered work,
+but the special requirements of the GNU Affero General Public License,
+section 13, concerning interaction through a network will apply to the
+combination as such.
 
   14. Revised Versions of this License.
-The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.
 
-Each version is given a distinguishing version number. If the Program specifies that a certain numbered version of the GNU General Public License “or any later version” applies to it, you have the option of following the terms and conditions either of that numbered version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of the GNU General Public License, you may choose any version ever published by the Free Software Foundation.
+  The Free Software Foundation may publish revised and/or new versions of
+the GNU General Public License from time to time.  Such new versions will
+be similar in spirit to the present version, but may differ in detail to
+address new problems or concerns.
 
-If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Program.
+  Each version is given a distinguishing version number.  If the
+Program specifies that a certain numbered version of the GNU General
+Public License "or any later version" applies to it, you have the
+option of following the terms and conditions either of that numbered
+version or of any later version published by the Free Software
+Foundation.  If the Program does not specify a version number of the
+GNU General Public License, you may choose any version ever published
+by the Free Software Foundation.
 
-Later license versions may give you additional or different permissions. However, no additional obligations are imposed on any author or copyright holder as a result of your choosing to follow a later version.
+  If the Program specifies that a proxy can decide which future
+versions of the GNU General Public License can be used, that proxy's
+public statement of acceptance of a version permanently authorizes you
+to choose that version for the Program.
+
+  Later license versions may give you additional or different
+permissions.  However, no additional obligations are imposed on any
+author or copyright holder as a result of your choosing to follow a
+later version.
 
   15. Disclaimer of Warranty.
-THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
+APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
+HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
+OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
+IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
 
   16. Limitation of Liability.
-IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
 
   17. Interpretation of Sections 15 and 16.
-If the disclaimer of warranty and limitation of liability provided above cannot be given local legal effect according to their terms, reviewing courts shall apply local law that most closely approximates an absolute waiver of all civil liability in connection with the Program, unless a warranty or assumption of liability accompanies a copy of the Program in return for a fee.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
 
                      END OF TERMS AND CONDITIONS
 
             How to Apply These Terms to Your New Programs
 
-If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
 
-To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
 
-     ConfigKit
-     Copyright (C) 2026  eric
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
 
-     This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
 
-     This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
 
-     You should have received a copy of the GNU General Public License along with this program.  If not, see <http://www.gnu.org/licenses/>.
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 Also add information on how to contact you by electronic and paper mail.
 
-If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode:
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
 
-     ConfigKit  Copyright (C) 2026  eric
+    <program>  Copyright (C) <year>  <name of author>
     This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
-     This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
 
-The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, your program's commands might be different; for a GUI interface, you would use an “about box”.
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
 
-You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see <http://www.gnu.org/licenses/>.
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<https://www.gnu.org/licenses/>.
 
-The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read <http://www.gnu.org/philosophy/why-not-lgpl.html>.
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<https://www.gnu.org/licenses/why-not-lgpl.html>.

README.md

@@ -5,18 +5,26 @@ An opinionated Go library for applications that use YAML configuration files.
 Linux is release-validated; macOS support is best-effort. See the
 [platform support policy](docs/policy/release.md#platform-support) for details.
 
-ConfigKit's initial loader accepts one strict YAML
-source (or explicit defaults-only input), optionally traverses explicit path and glob
-imports, strictly composes their disjoint definitions, applies typed defaults, and
-can overlay one selected fixed overlay and check an application schema version. It
-returns detached metadata and safe diagnostics.
+ConfigKit's simple `Load` accepts one strict YAML source (or explicit
+defaults-only input). `LoadLayered` additionally combines explicit lower-priority
+source groups with an independently selected main input. Sources can be host
+files, native `fs.FS` files, labeled bytes or readers, or snapshotted mappings.
+Enabled path and glob imports compose strictly within their group; precedence
+between groups combines mappings recursively. Both entry points return a complete,
+validated configuration with detached metadata and safe diagnostics.
 Applications may perform audited defaulting and normalization through a resolver,
 then report domain errors and warnings through read-only semantic validation. Secret
 tags and a path predicate drive deterministic redacted effective YAML. Typed secret
 references resolve environment names before validation, and explicit
 secrets-directory bootstrap is available as a separate startup operation. Explicit
 CLI override records apply above overlays. Value explanation is available through
-detached structured records and deterministic redacted output.
+detached structured records and deterministic redacted output. Built-in
+`extends` relationships let a mapping inherit from a named sibling during the
+same load, retaining explicit presence and source/derivation history. The root
+`Diff` API compares frozen effective values from two successful same-type loads;
+the [configuration contract](docs/config.md#effective-configuration-diffs) and
+[consumer guide](docs/consumers/README.md#effective-configuration-diffs) describe
+its safe changed-region records and maintained workflows.
 
 ```go
 type Config struct {
@@ -33,10 +41,40 @@ func ReadConfig(path string) (Config, error) {
 }
 ```
 
+## Design philosophy
+
+ConfigKit favors one coherent, predictable configuration model over flexibility
+for reproducing each consumer's conventions. Its goal is to let applications
+load once, inspect where final values came from, and distinguish explicit values
+from omissions with minimal configuration bookkeeping. Applications retain their
+domain rules and execution responsibilities.
+
+Loading is synchronous and complete on return. Applications normally load once
+near startup and then use the final configuration and metadata; ConfigKit stays
+inactive until explicitly called again. Applications may initiate additional
+loads as needed. See the [loading lifecycle](docs/config.md#completion-and-resource-lifetime)
+for completion and ownership details.
+
+Within a configuration tier, independent contributions compose strictly: shared
+mappings combine, while overlapping non-mapping definitions are errors. Between
+precedence tiers, mappings combine recursively, retaining omitted lower-priority
+children; other values replace lower-priority values. The main document and its
+imports form one strictly composed tier. Explicit underlay groups sit below it
+and above typed defaults; a selected overlay and explicit CLI overrides sit
+above it. The [configuration contract](docs/config.md#strict-document-composition)
+owns the detailed combination rules and input-specific constraints.
+
+The [architecture goals](docs/policy/architecture.md#success-criteria) explain
+these priorities and how genuine new needs can justify extending the shared
+model. The [deferred scope](docs/roadmap/deferred.md) records deliberate exclusions.
+
 The [configuration contract](docs/config.md) describes implemented behavior. The
-[consumer adoption guide](docs/consumers/README.md) provides maintained Narratio-,
-Notarius-, WeatherReporter-, and Seriatim-shaped examples and migration guidance.
-The [deferred scope](docs/roadmap/deferred.md) records deliberate exclusions. See the
-[development guide](docs/development.md) for repository orientation and the
+[consumer adoption guide](docs/consumers/README.md) provides maintained simple
+native-filesystem, layered, and sibling-inheritance examples alongside the migration shapes.
+See the [development guide](docs/development.md) for repository orientation and the
 [release procedure](docs/release.md) for maintainer-only candidate and publication
 steps.
+
+ConfigKit code is licensed under the [LGPL v3.0 or later](COPYRIGHT).
+
+[Open Source attribution notices](docs/oss/attribution.md) are available.

conversion.go

@@ -43,16 +43,36 @@ func toEngineLocations(values []Location) []engine.Location {
 	return result
 }
 func fromEngineSourceInfo(value engine.SourceInfo) SourceInfo {
-	return SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: Layer(value.Layer), ImportedFrom: fromEngineLocationPointer(value.ImportedFrom)}
+	return SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: Layer(value.Layer), Kind: SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: fromEngineLocationPointer(value.ImportedFrom)}
 }
 func toEngineSourceInfo(value SourceInfo) engine.SourceInfo {
-	return engine.SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: engine.Layer(value.Layer), ImportedFrom: toEngineLocationPointer(value.ImportedFrom)}
+	return engine.SourceInfo{ID: value.ID, Label: value.Label, Path: value.Path, Layer: engine.Layer(value.Layer), Kind: engine.SourceKind(value.Kind), UnderlayIndex: value.UnderlayIndex, ImportedFrom: toEngineLocationPointer(value.ImportedFrom)}
 }
 func fromEngineOrigin(value engine.Origin) Origin {
-	return Origin{Layer: Layer(value.Layer), Locations: fromEngineLocations(value.Locations), Operation: ChangeKind(value.Operation)}
+	result := Origin{Layer: Layer(value.Layer), Locations: fromEngineLocations(value.Locations), Operation: ChangeKind(value.Operation)}
+	if value.Inheritance != nil {
+		result.Inheritance = make([]InheritanceStep, len(value.Inheritance))
+		for index, step := range value.Inheritance {
+			result.Inheritance[index] = InheritanceStep{
+				From: fromEnginePath(step.From), To: fromEnginePath(step.To), Reference: fromEnginePath(step.Reference),
+				Locations: fromEngineLocations(step.Locations),
+			}
+		}
+	}
+	return result
 }
 func toEngineOrigin(value Origin) engine.Origin {
-	return engine.Origin{Layer: engine.Layer(value.Layer), Locations: toEngineLocations(value.Locations), Operation: engine.ChangeKind(value.Operation)}
+	result := engine.Origin{Layer: engine.Layer(value.Layer), Locations: toEngineLocations(value.Locations), Operation: engine.ChangeKind(value.Operation)}
+	if value.Inheritance != nil {
+		result.Inheritance = make([]engine.InheritanceStep, len(value.Inheritance))
+		for index, step := range value.Inheritance {
+			result.Inheritance[index] = engine.InheritanceStep{
+				From: toEnginePath(step.From), To: toEnginePath(step.To), Reference: toEnginePath(step.Reference),
+				Locations: toEngineLocations(step.Locations),
+			}
+		}
+	}
+	return result
 }
 func fromEngineDiagnostic(value engine.Diagnostic) Diagnostic {
 	return Diagnostic{Code: Code(value.Code), Severity: Severity(value.Severity), Path: fromEnginePath(value.Path), Message: value.Message, Primary: fromEngineLocationPointer(value.Primary), Related: fromEngineLocations(value.Related)}

development_check_test.go

@@ -0,0 +1,110 @@
+package configkit
+
+import (
+	"bytes"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestDevelopmentCheckHandlesFilenamesAndStopsOnFailure(t *testing.T) {
+	for _, scriptName := range []string{"check.sh"} {
+		t.Run(scriptName, func(t *testing.T) {
+			script, err := os.ReadFile("scripts/" + scriptName)
+			if err != nil {
+				t.Fatal(err)
+			}
+			for _, name := range []string{"success", "failed-go-check", "untracked-whitespace"} {
+				fail := name != "success"
+				t.Run(name, func(t *testing.T) {
+					root := t.TempDir()
+					bin := t.TempDir()
+					log := filepath.Join(bin, "gofmt.args")
+					calls := filepath.Join(bin, "go.calls")
+					for _, directory := range []string{"scripts", "nested"} {
+						if err := os.Mkdir(filepath.Join(root, directory), 0700); err != nil {
+							t.Fatal(err)
+						}
+					}
+					write := func(path string, data []byte, mode os.FileMode) {
+						t.Helper()
+						if err := os.WriteFile(path, data, mode); err != nil {
+							t.Fatal(err)
+						}
+					}
+					write(filepath.Join(root, "scripts", scriptName), script, 0700)
+					write(filepath.Join(root, "go.mod"), []byte("module example.invalid/check\n"), 0600)
+					write(filepath.Join(root, "go.sum"), nil, 0600)
+					write(filepath.Join(root, "obsolete.go"), []byte("package obsolete\n"), 0600)
+					files := []string{"nested/a space.go", "nested/a\nnewline.go"}
+					for _, file := range files {
+						write(filepath.Join(root, file), []byte("package example\n"), 0600)
+					}
+					if name == "untracked-whitespace" {
+						write(filepath.Join(root, "notes.md"), []byte("trailing space \n"), 0600)
+					}
+					// Isolate expensive toolchain commands: this test owns shell orchestration,
+					// not Go's compiler or the production tests exercised by the real checker.
+					write(filepath.Join(bin, "go"), []byte("#!/bin/sh\nprintf '%s/%s %s\\n' \"${GOOS-native}\" \"${GOARCH-native}\" \"$*\" >> \"$CONFIGKIT_TEST_GO_LOG\"\nif [ \"${CONFIGKIT_TEST_FAIL-}\" = yes ] && [ \"${1-}\" = test ]; then exit 37; fi\n"), 0700)
+					write(filepath.Join(bin, "gofmt"), []byte("#!/bin/sh\nprintf '%s\\000' \"$@\" > \"$CONFIGKIT_TEST_FMT_LOG\"\n"), 0700)
+					init := exec.Command("git", "init", "-q")
+					init.Dir = root
+					if out, err := init.CombinedOutput(); err != nil {
+						t.Fatalf("git init: %v %s", err, out)
+					}
+					// HEAD is required by the whitespace gate. Commit only the module baseline.
+					for _, args := range [][]string{{"add", "go.mod", "go.sum", "obsolete.go"}, {"-c", "user.name=Test", "-c", "user.email=test@example.invalid", "-c", "commit.gpgsign=false", "commit", "-qm", "Test fixture"}} {
+						cmd := exec.Command("git", args...)
+						cmd.Dir = root
+						if out, err := cmd.CombinedOutput(); err != nil {
+							t.Fatalf("git fixture: %v %s", err, out)
+						}
+					}
+					if err := os.Remove(filepath.Join(root, "obsolete.go")); err != nil {
+						t.Fatal(err)
+					}
+					cmd := exec.Command("/bin/bash", filepath.Join(root, "scripts", scriptName))
+					cmd.Env = append(os.Environ(), "PATH="+bin+string(os.PathListSeparator)+os.Getenv("PATH"), "CONFIGKIT_TEST_FMT_LOG="+log, "CONFIGKIT_TEST_GO_LOG="+calls)
+					// Log only targets selected by the script, not inherited host settings.
+					cmd.Env = append(cmd.Env, "GOOS=", "GOARCH=")
+					if name == "failed-go-check" {
+						cmd.Env = append(cmd.Env, "CONFIGKIT_TEST_FAIL=yes")
+					}
+					out, err := cmd.CombinedOutput()
+					if fail {
+						if err == nil {
+							t.Fatal("checker ignored failed Go test")
+						}
+					} else if err != nil {
+						t.Fatalf("checker: %v %s", err, out)
+					}
+					arguments, err := os.ReadFile(log)
+					if err != nil {
+						t.Fatal(err)
+					}
+					parts := bytes.Split(arguments, []byte{0})
+					for _, file := range files {
+						found := false
+						for _, arg := range parts {
+							if string(arg) == file {
+								found = true
+							}
+						}
+						if !found {
+							t.Fatalf("filename was split: %q", file)
+						}
+					}
+					invoked, err := os.ReadFile(calls)
+					if err != nil {
+						t.Fatal(err)
+					}
+					if fail && strings.Contains(string(invoked), "vet ./...") {
+						t.Fatal("checker continued after failed test")
+					}
+				})
+			}
+		})
+	}
+}

diff.go

@@ -0,0 +1,53 @@
+package configkit
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/engine"
+
+// Difference describes one canonical changed region between two successful
+// loads. Left is the earlier value and Right is the later value. Each snapshot
+// is detached; Present distinguishes absence from an explicit null, and
+// Redacted reports whether combined sensitivity affected that side. Mappings
+// are compared recursively, while sequences are reported as atomic values.
+// Records are ordered by canonical JSON pointer. HiddenChanged indicates that
+// a value concealed by combined sensitivity changed, without revealing which
+// concealed member changed.
+type Difference struct {
+	Path          Path
+	Left          ValueSnapshot
+	Right         ValueSnapshot
+	HiddenChanged bool
+}
+
+// Diff compares the frozen effective values of two successful loads of the
+// same configuration type. It does not reread sources, environment variables,
+// or rerun defaults or resolvers. Equal values produce no records even when
+// their sources, defaults, or resolution histories differ. Credential contents
+// are not compared: typed secret references compare by reference name only.
+//
+// Both results must be non-nil successful loads with inspection state; nil or
+// zero results return an error and no records. Load errors are returned by
+// Load, so callers must handle those before calling Diff. Snapshots are safe
+// for presentation but are not a complete reloadable configuration.
+func Diff[T any](left, right *Result[T]) ([]Difference, error) {
+	var leftInspection, rightInspection *engine.Inspection
+	if left != nil {
+		leftInspection = left.inspection
+	}
+	if right != nil {
+		rightInspection = right.inspection
+	}
+
+	values, err := engine.Diff(leftInspection, rightInspection)
+	if err != nil {
+		return nil, err
+	}
+	differences := make([]Difference, len(values))
+	for index, value := range values {
+		differences[index] = Difference{
+			Path:          fromEnginePath(value.Path),
+			Left:          fromEngineValueSnapshot(value.Left),
+			Right:         fromEngineValueSnapshot(value.Right),
+			HiddenChanged: value.HiddenChanged,
+		}
+	}
+	return differences, nil
+}

docs/adr/0007-source-backed-underlays.md

@@ -0,0 +1,118 @@
+# ADR 0007: Add Source-Backed Underlays with Consistent Recursive Precedence
+
+## Status
+
+Proposed draft for review. Completed planning and remediation records are
+preserved in Git history at `b604aa1` under `docs/roadmap/`; the
+[configuration contract](../config.md#layered-loading) owns current behavior.
+
+This extends [ADR 0001](0001-separate-composition-and-overlay.md) and preserves
+its distinction between strict composition and intentional overlay.
+
+## Date
+
+2026-09-29
+
+## Context
+
+Configuration-backed defaults provide lower-priority settings that applications
+can override. These sources carry imports, locations, and relative-resource
+contexts. Integrating them into ConfigKit's loading lifecycle gives their values
+the same presence tracking and inspectable provenance as other configuration
+inputs.
+
+The [architecture goals](../policy/architecture.md#success-criteria) prioritize
+one coherent, predictable configuration model with consistent rules across
+source representations and loading interfaces. Consumers should load once,
+receive a complete validated configuration, and inspect its final values' origins.
+Reducing integration code is important subject to those principles.
+
+## Decision
+
+### Add underlays to the existing precedence model
+
+Introduce explicitly supplied source-backed underlays between typed defaults
+and main configuration. The proposed precedence, from lowest to highest, is:
+
+**Typed application defaults < source underlays < strictly composed main
+configuration < selected overlay < explicit CLI overrides.**
+
+Typed defaults remain the Go-value baseline. Underlays supply lower-priority
+configuration with source provenance and, where applicable, imports and resource
+context. This ordering describes value precedence; typed defaults fill fields
+absent from the assembled input under the existing defaulting rules.
+
+Preserve strict composition of the main document and its imports before applying
+the resulting configuration over underlays. Independent contributions within an
+underlay tier also compose strictly. Explicitly ordered tiers express intentional
+precedence. Strict composition reports conflicting contributions within each tier.
+
+Across precedence tiers, reuse the existing recursive mapping-overlay semantics.
+Omitted children retain lower-priority input; existing scalar, sequence, null,
+and cross-kind replacement semantics remain unchanged. Reusing these operations
+keeps the combination rules consistent across the loading model.
+
+### Keep source representation separate from composition semantics
+
+Native `fs.FS` and programmatic inputs join the shared loading model.
+Equivalent inputs using the same
+operation have the same presence and precedence semantics regardless of medium.
+Programmatic values enter directly as labeled configuration sources.
+
+Retain the contributing source's identity and resource context through layering.
+A retained underlay value keeps its originating source's resource context.
+Consumers remain responsible for opening and interpreting their domain resources
+under the supplied filesystem's lifetime and access semantics. Exact
+context-access and input APIs remain roadmap decisions.
+
+### Deliver one final configuration and its inspection metadata
+
+Assemble effective input, then run the typed loading lifecycle once for the load.
+Preserve explicit zero,
+false, empty, and null input versus omission through configuration and metadata
+together.
+
+Extend existing provenance and history through underlays, defaults, and recorded
+application resolution. The returned result includes the completed combination,
+defaulting, and provenance work. Domain resolution and validation
+remain application-owned within the recorded lifecycle, consistent with
+[ADR 0003](0003-preserve-presence-and-resolution-history.md).
+
+Preserve the existing opt-in schema-version ownership, path protections, and
+check timing before precedence
+assembly and typed decoding. The roadmap owns how layered input identifies that
+owner when version checking is configured.
+
+Favor near-linear processing in input, retained history, and output size through
+incremental assembly of load-owned state. This is an engineering objective
+subordinate to clarity and semantic consistency. Preserve the root-owned facade and
+private implementation boundary established by [ADR 0006](0006-public-facade.md).
+
+## Alternatives considered
+
+- **Leave bundled defaults to consumers.** Avoids extending loading, but makes
+  consumers reproduce merging, presence tracking, and provenance reconciliation.
+  It does not meet the goal of obtaining a complete configuration in one load.
+- **Offer recursive or atomic combination per layer.** Makes both behaviors
+  explicit and could serve more consumers, but expands the semantic surface and
+  its interactions with presence and provenance without a demonstrated need that
+  outweighs a single consistent rule. A future concrete requirement may justify
+  reconsideration; this feature does not add the option.
+- **Flatten all documents into last-wins layers.** Simplifies ordering but turns
+  accidental main/import conflicts into overrides, weakening the intentional
+  distinction established by ADR 0001.
+
+## Consequences
+
+Consumers gain source-backed defaults within the shared loading lifecycle.
+Strict contributions and intentional precedence remain distinct, with consistent
+combination rules across source media. Presence and provenance remain
+part of the same loading lifecycle as the final values.
+
+Higher-priority mappings retain omitted lower-priority children, so application
+validation assesses the final combined configuration.
+
+Implementation must integrate source context and underlay history into the shared
+engine with attention to assembly and history-retention costs. Current behavior
+documentation will change when the feature lands. The roadmap retains ownership
+of the remaining API decisions.

docs/adr/0008-complete-on-return-loading.md

@@ -0,0 +1,82 @@
+# ADR 0008: Synchronous, Complete-on-Return Configuration Loading
+
+## Status
+
+Proposed draft for review. The
+[configuration contract](../config.md#completion-and-resource-lifetime) describes
+the implemented loading lifecycle and
+[source resource contexts](../config.md#source-resource-contexts).
+
+## Date
+
+2026-09-29
+
+## Context
+
+Applications need a clear point at which configuration is ready for use.
+Completing configuration processing and inspection metadata together gives
+applications a coherent result whose values and origins agree. A bounded loading
+lifecycle also makes resource ownership and startup failures straightforward to
+reason about.
+
+ConfigKit's [architecture goals](../policy/architecture.md#success-criteria)
+emphasize complete configuration, inspectable provenance, predictable behavior,
+and minimal consumer bookkeeping. Applications own their execution lifecycle and
+the timing of configuration acquisition.
+
+## Decision
+
+Make loading a synchronous, complete-on-return operation. Within each load,
+ConfigKit completes input assembly, defaulting, typed decoding, configured
+application resolution, secret lookup, validation, and snapshotting before
+returning the final configuration and metadata. Reported failures return
+diagnostics and a nil result.
+
+Release all library-owned loading handles before returning on success or failure.
+The resulting configuration and metadata are independently usable, caller-owned
+data with a retained inspection snapshot. Caller-supplied readers and filesystems
+remain under caller ownership and lifetime control.
+
+After the call completes, ConfigKit remains inactive until explicitly invoked
+again. Applications normally load once at or near startup and may initiate
+additional loads when needed. Each load follows the same lifecycle and source
+reuse contracts. Inspection, rendering, and separately requested operations run
+in response to explicit application calls.
+
+For the planned resource-context capability, retain source context as passive
+information or a borrowed caller-supplied filesystem. Subsequent resource access
+acquires its own handles with explicit ownership. This preserves a clear boundary
+between configuration completion and application resource use.
+
+These choices give consumers one completion boundary for configuration and
+provenance, predictable resource lifetimes, and control over when ConfigKit runs.
+Application callbacks remain responsible for their own effects and resources.
+
+## Alternatives considered
+
+- **Background watching or refresh.** Can automate responses to source changes,
+  but adds scheduling, shutdown, publication, and resource-lifetime concerns to
+  the library. Explicit calls keep those responsibilities with the application.
+- **Lazy completion after return.** Can defer work until particular values are
+  used, but moves configuration failures into application execution and makes
+  the readiness of values and metadata harder to establish.
+- **Transactional atomic loading.** Could imply simultaneous observation of all
+  external inputs and rollback of callback effects. Filesystems, environment
+  variables, and application callbacks do not share a transaction mechanism.
+  Synchronous completion provides the required usable-result boundary without
+  promising those external guarantees.
+- **Retain loading handles in results.** Can preserve open filesystem access,
+  but gives configuration results an ongoing resource lifecycle and cleanup
+  obligation. Passive source context supports later explicit resource access
+  while keeping loading resources scoped to the load call.
+
+## Consequences
+
+Applications can treat successful return as the point at which the final
+configuration and its metadata are ready for use. Startup loading remains simple,
+and additional loads have an explicit application-owned trigger.
+
+Configuration inspection operates on retained data. Resource access observes the
+backing resource when explicitly requested and has its own failure and ownership
+boundary. Implementations of new source kinds and layering features preserve the
+same completion boundary, including cleanup on reported load failures.

docs/adr/0009-sibling-inheritance.md

@@ -0,0 +1,76 @@
+# ADR 0009: Resolve Sibling Inheritance After Source Assembly
+
+## Status
+
+Accepted and implemented. The [configuration contract](../config.md#sibling-inheritance)
+owns current behavior. The completed implementation plan is preserved in Git
+history at `83c59ea`.
+
+## Date
+
+2026-09-29
+
+## Context
+
+Named configuration mappings often share settings while retaining individual
+overrides. ConfigKit already distinguishes strict composition of independent
+source contributions from recursive overlay between explicit precedence tiers.
+Doing another untracked merge in applications would lose omission, source
+context, and historical explanation information. A shared structural operation
+must preserve the same presence and provenance model as the rest of loading.
+
+Inheritance also interacts with source precedence, nested namespaces, version
+ownership, and secrets. Resolving it before source assembly would validate
+superseded references and copy values that a later tier should replace. Fully
+resolving nested relationships inside a reusable parent would bind them to the
+wrong sibling scope when copied into a child.
+
+## Decision
+
+Reserve `extends` as a built-in input key on mappings. A string names one exact
+sibling mapping; null cancels the relationship. Complete source acquisition,
+strict composition, selected overlay, and CLI precedence first. Then resolve
+each effective relationship in the parser-independent document tree, before
+typed defaults and decoding. A child recursively overlays its own payload on
+its resolved parent using the existing mapping-combination rules. This creates
+neither a source tier nor a configurable merge strategy.
+
+Resolve enclosing sibling chains before nested relationships. Reuse memoized
+parent templates with unresolved nested instructions; evaluate those instructions
+at each destination. Preserve original source applications separately from
+structural transfer/control evidence. Effective origins carry concrete
+source-to-destination derivation, and inspection projects relevant inheritance
+events between source replay and typed reconciliation. Inherited input counts as
+explicit presence, while direct replacement retains its own effective origin.
+
+The existing main-input version check remains at its original point. Reject
+structural contributions or directive consumption affecting its protected path.
+Classify sensitivity at corresponding source and destination paths and retain
+historical redaction. Secret references remain unresolved input until the
+existing lookup point. Keep all machinery behind the root-owned public facade.
+
+## Alternatives considered
+
+- **Application-owned inheritance after `Load`.** Leaves every consumer to
+  recreate ordering, presence, provenance, error reporting, and redaction after
+  ConfigKit has already declared a result complete.
+- **Resolve each source before precedence assembly.** Makes superseded
+  references active and can copy a parent definition before higher-priority
+  input finishes changing it.
+- **Resolve nested relationships in a cached parent subtree.** Shares more
+  ready-made output but binds nested sibling lookups to the parent's scope,
+  giving incorrect values at a child destination.
+- **Registered collections, configurable keywords, or multiple-parent merge
+  modes.** Increase public setup and semantic branches without a requirement
+  that warrants diverging from one structural rule.
+
+## Consequences
+
+Consumers can load a configuration once and inspect direct, inherited,
+defaulted, and application-derived values through the existing result surfaces.
+`extends` is no longer an ordinary application field, so existing configurations
+using that key as data must migrate. Expanding many children can materialize
+substantially more output than the input contains; projection and retained-history
+budgets bound that work without charging copies as newly acquired sources.
+Applications still own domain validation, entry selection, and runtime resource
+use.

docs/adr/0010-effective-configuration-diff.md

@@ -0,0 +1,99 @@
+# ADR 0010: Compare Frozen Effective Configuration Snapshots
+
+## Status
+
+Accepted and implemented. The
+[configuration contract](../config.md#effective-configuration-diffs) owns current
+behavior. Completed planning and remediation records are preserved in Git
+history at `1c2610c` under `docs/roadmap/`.
+
+## Date
+
+2026-10-09
+
+## Context
+
+Applications need to inspect how two completed configurations differ, including
+the effective impact of an overlay after defaults and application resolution.
+ConfigKit already retains canonical typed inspection snapshots, semantic paths,
+presence information, and sensitivity classification for successful loads.
+
+Extending this inspection model lets applications share configuration comparison
+and safe disclosure while keeping their presentation and domain interpretation.
+The operation should follow ConfigKit's existing equality, ownership, and secret
+boundaries so consumers can reason about loading and inspection consistently.
+
+## Decision
+
+Provide structured effective-value comparison between successful results of the
+same configuration type. Compare their retained canonical inspection snapshots
+through the root-owned public facade. Each comparison uses the completed load
+state and returns detached records with a semantic path and safe left and right
+value snapshots.
+
+Use canonical value equality and the existing structural comparison rules.
+Mappings compare recursively, sequences report atomic changes, and scalar or
+kind replacements report the affected region. Presence distinguishes additions
+and removals from explicit null. Equal effective values produce an empty diff;
+source attribution, explicit-input status, and history remain separately
+inspectable through each result. Canonical pointer ordering makes reporting
+deterministic.
+
+Detect changes before masking values. Apply sensitivity from both loads to both
+returned sides, including historical and dependency-derived classification.
+Preserve public content where the shapes permit corresponding sensitive regions
+to be identified. When a structural replacement prevents safe correspondence,
+mask the complete reported replacement region on both present sides. Absence
+retains its own representation.
+
+Keep masking and hidden-change information distinct. Each record indicates
+whether an actual change is withheld by redaction, allowing applications to
+display a changed-sensitive-value indicator even when safe snapshots render
+identically. Public-only changes alongside unchanged masked content retain that
+distinction.
+
+Typed secret references participate through their configured environment names.
+Resolved credential bytes remain behind the existing explicit secret accessor.
+The comparison uses the same frozen inspection boundary as effective rendering
+and explanation.
+
+ConfigKit owns comparison and paired safe disclosure. Applications own labels,
+layout, and rendering of the returned changed regions. Reusing the existing
+snapshot representation and engine boundary keeps the extension consistent with
+the library's established inspection model.
+
+## Alternatives considered
+
+- **Compare input text or separately rendered YAML.** Supports familiar textual
+  displays, but input text precedes composition and resolution, and independently
+  redacted output can hide changes or apply different disclosure rules. Canonical
+  comparison with paired masking represents the completed configuration.
+- **Leave comparison to applications using returned Go values.** Offers direct
+  access to application types, but duplicates structural and redaction machinery
+  and observes caller mutation. Retained inspection snapshots preserve the
+  completed load's values and established ownership boundary.
+- **Align sequence elements and return context-rich rendering data.** Enables
+  finer-grained displays, but requires additional correspondence and disclosure
+  rules. Atomic sequence records and existing value snapshots satisfy the
+  selected changed-region workflows with a smaller public model.
+- **Compare resolved credentials or retain credential-derived tokens.** Could
+  detect rotation behind an unchanged reference, but expands retained sensitive
+  state and introduces a separate trusted comparison mechanism. Reference-name
+  comparison preserves the existing inspection boundary.
+
+## Consequences
+
+Applications can compare independently loaded configurations using ConfigKit's
+shared equality, presence, and disclosure rules. Existing composition and load
+semantics continue to determine the values being compared. Caller mutation of a
+returned configuration leaves its retained comparison snapshot unchanged.
+
+Paired redaction can conceal public content within a structurally incompatible
+replacement. The changed region and hidden-change indicator still communicate
+its effective impact. Credential rotation behind an unchanged environment name
+produces equal reference values for comparison.
+
+The implementation requires operation-local disclosure preparation and focused
+coverage for presence, shape changes, historical sensitivity, and detached
+results. Applications retain control of presentation; changed-region records
+supply the safe data needed for that responsibility.

docs/config.md

@@ -12,13 +12,21 @@ ConfigKit exposes opaque sources through these constructors:
 | Constructor | Current contract |
 | --- | --- |
 | `File(path)` | Requires a nonempty path, opens a regular file for each consumption, and closes only the library-owned file handle. Missing, inaccessible, unreadable, directory, and other nonregular inputs fail. |
+| `NewFilesystem(label, filesystem).File(name)` | Borrows a caller-owned `fs.FS` under a safe nonempty display label. `name` must be a non-root slash-relative `fs.ValidPath`. Each load reopens the selected regular file; copies of the `Filesystem` preserve namespace identity. Separate constructors create separate namespaces even for the same label or filesystem. |
 | `Bytes(label, data)` | Requires a nonempty, non-secret diagnostic label and snapshots bytes during construction. Later caller mutation cannot affect the source. |
 | `Reader(label, input)` | Requires a nonempty, non-secret label and non-nil reader. The reader is claimed once across all copies of the source, consumed but never closed, and cannot be reused. |
+| `Mapping(label, values)` | Requires a safe nonempty label and a non-nil root `map[string]any`. Snapshots supported built-in values into an owned mapping document at construction; `{}` is valid. Constructor conversion failures become safe deferred diagnostics. |
 | `NoSource()` | Selects defaults-only loading, creates no source record, and marks no paths explicit. |
 
 The zero `Source` is invalid. Constructor argument failures are retained inside the
 opaque source and become safe structured diagnostics when the source is consumed.
-File sources can observe file changes between consumptions; byte sources cannot.
+File and filesystem sources can observe changes between consumptions; byte and
+mapping sources cannot. Mapping accepts nil, built-in string, boolean, signed or
+unsigned integer, finite float, `[]any`, and `map[string]any` values recursively.
+Typed nil maps and slices represent explicit null; empty collections remain empty.
+Keys are sorted for stable provenance, integer decimal spelling stays exact, and
+cycles or unsupported types are rejected without exposing values. Mapping input
+uses the same directive and version rules as YAML, not CLI override restrictions.
 
 Each source is limited to 8 MiB. A load using imports or an active overlay is limited
 to 64 MiB across all accepted sources, 1,024 accepted sources, 64 active import
@@ -26,6 +34,8 @@ edges, and 1,000,000 input nodes in aggregate. Glob expansion separately permits
 1,000,000 visited directory entries, including nonmatches. Reads request at most the
 remaining allowance plus one byte so excess data is rejected rather than silently
 truncated. Data returned together with a read error is not accepted as partial input.
+For mapping sources, key and string bytes use the same accounting as programmatic
+overrides, and the converted tree uses the same node and depth bounds.
 
 ## Single-source loading
 
@@ -44,13 +54,14 @@ The current lifecycle is:
 ```text
 validate options → parse the selected source → traverse/compose enabled imports
   → build schema → check the optional application version → apply a selected overlay
-  → apply explicit CLI overrides → create and clone defaults → strictly decode
+  → apply explicit CLI overrides → expand sibling inheritance
+  → create and clone defaults → strictly decode
   → recorded application resolution
   → resolve typed secret references → semantic validation → classify secrets
   → snapshot configuration and metadata
 ```
 
-`File`, `Bytes`, and `Reader` each contribute the first base source with deterministic
+`File`, `Filesystem.File`, `Bytes`, `Reader`, and `Mapping` each contribute the first base source with deterministic
 ID `source-1`; enabled imports continue in depth-first order as `source-2` onward.
 `NoSource` skips parsing, supplies an absent version to a configured version checker,
 contributes no source, and continues through defaults, decode, and projection. A
@@ -66,11 +77,166 @@ disabled.
 
 Every source, options, schema, version, defaults, decode, resolution, or validation
 failure returns a nil result and a `*LoadError`. Successful results contain caller-owned `Config`, detached
-`Metadata`, a defensive source list, and defensive nonfatal diagnostics. Mutating
+`Metadata`, a defensive source list, and defensive nonfatal diagnostics. Each source
+record's `Kind` identifies file, filesystem, bytes, reader, mapping, or CLI acquisition
+independently of its precedence `Layer`. Host file paths remain canonical;
+filesystem paths are slash-relative within their namespace; synthetic inputs have
+no path. Mutating
 the returned configuration or inspection records cannot alter ConfigKit's private
 snapshot. ConfigKit performs no logging. `Result.WriteEffective` exposes only the
 frozen redacted projection.
 
+### Completion and resource lifetime
+
+`Load` is synchronous and complete on return. Success means input assembly,
+defaulting, typed decoding, configured application resolution, secret lookup,
+validation, and configuration/metadata snapshotting have completed. A reported
+load failure returns diagnostics and a nil result, as described above.
+
+Before returning on success or failure, ConfigKit closes the filesystem handles
+it opened for loading, including files and directory/root handles. Caller-owned
+filesystems are borrowed, never closed. The returned
+configuration and inspection metadata require no close operation and remain
+usable independently of the source files. A supplied `Reader` remains
+caller-owned and is never closed by ConfigKit.
+
+ConfigKit performs no background work after returning: it does not watch inputs,
+refresh secrets, or mutate published results. Inspection and rendering run only
+when explicitly called and use the retained snapshot. Applications normally load
+once near startup, but may explicitly load again using sources that satisfy their
+documented reuse rules. Additional loads produce independently owned results.
+
+Completion is not a transaction over external state. Files and environment
+variables are observed during processing rather than as one simultaneous external
+snapshot. ConfigKit does not roll back application callback effects or manage work
+or resources created by application callbacks. The explicit secrets-directory
+bootstrap remains a separate operation invoked by the application.
+
+## Layered loading
+
+`LoadLayered[T](main, options...)` accepts zero or one `LayeredOptions[T]` value.
+Its `main` is an `Input` containing a source, import opt-in, and optional
+filesystem context. A zero source is invalid; `Input{Source: NoSource()}` omits
+the main document. Each `Underlay` contains inputs that compose strictly as
+one tier, including each input's enabled imports. Empty groups contribute
+nothing. Underlay groups are listed from lowest to highest priority. Repeating
+a file within one group remains an import/repetition error; the same reusable
+source may be selected again in another tier.
+
+ConfigKit acquires and composes the main input first, constructs the schema,
+and checks the optional application version from the main tier only. Underlay
+roots cannot claim that version path (`underlay.version`); imported fragments
+retain `import.version` protection. It then acquires and composes underlay
+groups in order, recursively combines the groups, and applies the main tier,
+one selected overlay, and explicit CLI overrides. Scalar, sequence, and null
+definitions replace lower values between tiers; mappings combine recursively.
+Syntax and strict-composition errors inside a tier cannot be masked by a
+higher-priority tier. Defaults, typed decoding, resolution, secret lookup,
+validation, and publication run once on the final assembled input.
+
+An active layered overlay requires `LayeredOptions.OverlayFilesystem` explicitly,
+independent of the main source's namespace. Its name and root-relative lookup
+follow the selected-overlay rules below; inactive overlays ignore the context.
+The same path in a different filesystem namespace is not a repeated file.
+Ordinary `Load` continues to infer its selected-overlay context from its source.
+
+Source IDs follow acquisition order—main and its depth-first imports, then each
+underlay tier's inputs and depth-first imports, then overlay and CLI—even though
+history follows precedence order. Source `Layer` describes precedence while
+`Kind` describes acquisition. Underlay records use `LayerUnderlay` and a
+one-based `UnderlayIndex` giving their original tier position; all other records
+have index zero. Explicit empty groups and `NoSource` consume no source ID.
+Byte, source, node, and glob-entry budgets are shared across the load, while
+the existing per-source and import-depth bounds still apply.
+
+The maintained [layered consumer example](consumers/README.md#native-filesystem-and-layered-input)
+shows a main host file, two underlay tiers, embedded defaults, a mapping source,
+and explicit resource access without a second configuration pass.
+
+### Sibling inheritance
+
+Any input mapping may declare `extends: name` to inherit from the mapping named
+`name` among its immediate siblings. Names are exact keys, including the empty
+string; lookup does not search ancestor or unrelated scopes. The document root
+and a sequence-element mapping have no sibling namespace for their own string
+relationship, though mappings nested inside them can have one. An `extends`
+value of null cancels a relationship without looking up a parent. Other value
+types, missing or non-mapping parents, and cycles fail the load with value-free
+diagnostics.
+
+`extends` is reserved structural input across ordinary and layered loads, every
+source kind, imports, selected overlays, and explicit CLI overrides. It is
+consumed before typed decoding and need not appear in the application's Go
+types. Applications previously using `extends` as an ordinary YAML field must
+rename that key and its field. No collection registration or keyword setup is
+required.
+
+ConfigKit completes strict composition within tiers and precedence assembly
+between tiers before resolving relationships. An omitted `extends` retains a
+lower-priority relationship; `extends: null` cancels it while preserving the
+child's own settings. Superseded references are historical evidence, not active
+relationships to validate. Each child recursively overlays its own settings
+over its resolved parent: explicit zero, false, empty string, sequence, and
+null values replace inherited values, while mappings combine recursively.
+The child retains its key. All effective relationships resolve, including
+entries the application may never select for execution. Partial parent
+definitions undergo normal typed decoding and application-owned validation,
+not a special structural exemption.
+
+Enclosing relationships resolve first. Nested instructions carried by a parent
+are then resolved separately at each destination, so a child's nested sibling
+override can change an inherited nested relationship. Typed defaults and
+application-derived values do not start another inheritance pass. Inherited
+input counts as explicit presence for `Metadata.IsExplicit`,
+`Resolution.IsExplicit`, and absence-based resolver defaults, even for false,
+zero, empty, or null input. `Origin.Inheritance` distinguishes it from a direct
+declaration with concrete source, destination, and controlling directive paths.
+A direct child replacement has its own effective origin; displaced ancestry
+remains inspectable in history. An inherited relative resource keeps the
+declaring source's passive `SourceContext`, not the child's directory.
+
+The optional schema-version checker still runs once against the main input at
+its original point. Inheritance cannot add, replace, or consume the protected
+version path; an unrelated relationship inside an enclosing mapping does not
+invalidate an unchanged version. Expansion uses the decoded projection
+node/depth limits without charging copied values as acquired sources;
+retained evidence uses the existing history-node limit. See the
+[consumer example](consumers/README.md#named-sibling-inheritance) for one-load
+usage.
+
+### Source resource contexts
+
+`Result.SourceContext(id)` and callback-scoped `Resolution.SourceContext(id)`
+return a passive `ResourceContext` for a source with filesystem context, or
+false for unknown and contextless IDs. The result query is nil-safe; the
+resolution query follows the callback's lifetime guard. Queries perform no
+filesystem access. Obtain IDs from exact origin locations rather than guessing
+one context for a composite origin or an application-derived value.
+
+`ResourceContext.FS` is rooted at the source's configured root, and `BaseDir`
+is its slash-relative directory inside that root. This includes imported
+sources, selected overlays, host files even without enabled imports, native
+files, and byte, reader, or mapping inputs with an explicit context. CLI and
+contextless synthetic inputs have no resource context. To open a relative
+resource, join `BaseDir` with the application resource name using slash-path
+operations and open that path through `FS`.
+
+For a resource-valued field, query `result.Metadata().Lookup(fieldPath)`, inspect
+its origin locations, choose a source ID only when the application's policy can
+identify the intended contributor, then call `result.SourceContext(id)`. The
+context belongs to that source even when the field survived from a lower-priority
+underlay. A composite origin has no automatically selected resource base.
+
+Native contexts borrow the caller's filesystem within the configured root.
+Host contexts retain a canonical root path, not an open root handle; each
+resource `Open` checks containment and opens a fresh handle that the returned
+file's `Close` releases. Applications own and close the files they open.
+Resources are application data, not configuration input, and do not consume
+configuration byte or node budgets. The loaded configuration and metadata stay
+usable if source files later change or disappear. Later resource operations
+observe the backing filesystem then and may fail if files move, disappear, or
+a caller-owned filesystem closes. This is not a snapshot or universal sandbox.
+
 ### Application schema versions
 
 `VersionRule` declares a semantic path and non-nil checker. Before defaults or typed
@@ -108,8 +274,9 @@ successful result. Callback panics remain programming failures.
 `Result.Metadata` describes every final canonical path. Queries retain explicit
 key/index distinctions using actual effective and historical container shapes;
 only parsed numeric tokens are resolved contextually. A descendant that never
-existed is not historical merely because an atomic ancestor was replaced. Exact surviving input paths
-are explicit and have a base or overlay origin; omitted paths have a default origin.
+existed is not historical merely because an atomic ancestor was replaced. Exact
+surviving direct or inherited input paths are explicit and retain their source
+layer and derivation; omitted paths have a default origin.
 An explicit parent mapping does not make omitted children explicit. Null state and
 available source coordinates are retained. `History` records the Go-zero or application
 default baseline followed by exact base or overlay definitions, including
@@ -124,8 +291,9 @@ domain ownership rules remain application responsibilities. The method's Go
 documentation in [metadata.go](../metadata.go) defines enumeration and path-ordering
 details.
 
-History is chronological: a path can have a synthetic default record, its exact base
-definition, an overlay replacement, and a recorded resolution operation. A descendant
+History is chronological: a path can have a synthetic default record, its exact
+base definition, an overlay replacement, structural inheritance, and a recorded
+resolution operation. A descendant
 query uses the ancestor's path for a whole-container or whole-sequence replacement;
 the descendant can therefore retain history after `Lookup` stops reporting it as
 effective. Equal-valued input is still an explicit base definition, while a derived
@@ -141,6 +309,15 @@ operation. Reasons and dependency paths retain first occurrence order with dupli
 removed. Source records remain ordered as selected source, depth-first imports, the
 selected overlay, then synthetic CLI records in supplied order.
 
+Inheritance adds no acquired source or precedence layer. `Origin.Inheritance`
+lists concrete transfers from contributor paths through destinations, with
+each step's controlling `extends` path and location. `History` retains source
+applications at their original paths and projects parent contributions, child
+replacements, and consumed controls at destination paths after source assembly
+and before typed decoding. `HistoricalPaths` includes consumed directives and
+displaced inherited descendants. A control can have history even when `Lookup`
+reports it absent. Queries return detached records.
+
 ### Secret classification and effective rendering
 
 Secret classification combines `configkit:"secret"` field tags with the optional
@@ -156,6 +333,11 @@ Propagation reaches a fixed point, so record/map iteration order does not affect
 result and dependency cycles terminate. A dependency on a classified descendant or
 on a container that contains one is treated conservatively as sensitive. Applications
 must classify secret-producing transformations whose dependencies are not recorded.
+Inherited value and control paths link source and destination classifications
+through chains and nested copies; unrelated sibling fields are not classified
+merely because another field is secret. Displaced historical evidence remains
+redacted. Secret references are copied only as unresolved names; credential
+lookup stays after decoding and application resolution.
 
 `Result.WriteEffective` writes the frozen resolved snapshot as deterministic redacted
 YAML. Mappings sort by decoded key string, sequences retain order, indentation is two
@@ -190,7 +372,10 @@ parser values. `Present=false` with empty YAML means absence or deletion; explic
 null is present and renders as `null`. The history starts with the canonical typed
 application-default or Go-zero baseline, then includes only relevant base sources in
 traversal order, the selected overlay, supplied CLI records, and audited net
-resolution operations. Overlapping parent/child resolution records are rendered
+resolution operations. Structural inheritance events appear after source replay
+and before typed reconciliation, including consumed or canceled `extends` input
+and equal-valued child replacements. Overlapping parent/child resolution records
+are rendered
 as one net subtree event with combined reasons and dependencies, rather than
 pretending that the recorded assignments ran in lexical order. Explicit equal
 writes remain visible. When input replay
@@ -238,6 +423,46 @@ an explicit `LoadSecretsDir` bootstrap, then use `WriteDiagnostics`,
 ConfigKit does not infer bootstrap outcomes from a secret reference and does not
 bundle these independent reports into a command framework.
 
+## Effective configuration diffs
+
+`configkit.Diff(left, right)` compares two completed successful loads of the same
+configuration type. `left` is the earlier state and `right` is the later state.
+Applications load both sides with the sources and options they choose; to inspect
+an overlay's impact, load the same base once without the overlay and once with it
+selected. The [consumer guide](consumers/README.md#effective-configuration-diffs)
+links to maintained examples. The public API returns safe changed-region records;
+applications own their presentation.
+
+Comparison uses the frozen canonical effective inspection values, not mutable
+`Result.Config`, source text, provenance, or history. It does not reopen sources,
+reread environment variables, or rerun defaults or callbacks. Equal effective
+values produce no records even when sources, explicit-input status, defaults, or
+resolution histories differ; for example, explicit zero and defaulted zero are
+equal. Mappings compare recursively, sequences compare atomically, and scalar,
+kind, or null/container replacements report their affected region. Records use
+the existing comparator's canonical pointer ordering. `Present=false` with empty
+YAML means absence; explicit null is present and renders as `null`.
+
+Before either side is returned, sensitivity from both loads is applied to both
+snapshots, including historical and dependency-derived classification. A
+classification on either side protects the corresponding values on both sides.
+`Redacted` reports masking, while `HiddenChanged` reports an actual concealed
+value or presence change; it is not implied merely by redaction. A structural
+replacement that prevents safe correspondence masks the complete reported
+region, and an absent side remains absent. Record membership establishes that a
+change occurred even when safe snapshots render identically. Literal
+marker-looking input is ordinary input; consumers use the record fields rather
+than interpreting marker text.
+
+Typed secret references contribute their configured environment names to the
+frozen comparison, never resolved credential bytes. Credential rotation behind
+an unchanged reference is therefore not detected. The records contain changed
+regions only, not paired full-document context; independently calling
+`WriteEffective` on each result does not create cross-protected context. Nil or
+uninitialized results return an error with no records. A failed `Load` retains
+its normal nil-result/`LoadError` behavior, which callers handle before invoking
+`Diff`.
+
 ## Typed secret references
 
 Credential-bearing fields use the opaque `Secret` type and store an environment
@@ -442,8 +667,8 @@ input document.
 `[a-zA-Z0-9_-]+`; they are not paths. The empty name and exact lowercase name
 `default` select no overlay, and `overlays/default.yml` is never loaded implicitly.
 Every other name resolves only to `<configuration-root>/overlays/<name>.yml`, with
-no extension fallback, case normalization, inheritance, or overlay stack. A selected
-missing overlay is an error.
+no extension fallback, case normalization, inheritance between overlay files,
+or overlay stack. A selected missing overlay is an error.
 
 The overlay is a separately policy-validated YAML mapping. It is loaded after the
 base import graph is composed and its schema version is checked, but before defaults
@@ -557,18 +782,25 @@ walk failures, matched directories, invalid documents, escapes, and repeated tar
 remain errors. Final candidates are bounded by the remaining source limit before
 their contents are accumulated.
 
-The configuration root defaults to a selected file's canonical directory. A file
-source may supply `FilesystemContext.Root` to widen that root but may not supply
-`BaseDir`. Whenever byte and reader sources supply filesystem context, they must
-provide both `BaseDir` and `Root` as absolute existing directories; their diagnostic
-labels, including URI-looking labels, never imply filesystem paths. The base and
-selected file must be inside the canonical root. Nonregular targets are rejected before opening to avoid blocking on FIFOs;
+For host files, the configuration root defaults to the selected file's canonical
+directory. A host file may supply `FilesystemContext.Root` to widen that root but
+may not supply `BaseDir` or a native `FS`. Byte, reader, and mapping inputs with host context
+must provide both `BaseDir` and `Root` as absolute existing directories.
+For `Filesystem.File`, the default root is `.` in its namespace and the base is
+the selected file's directory. An explicit context must use the same `Filesystem`
+namespace and may set `Root` to contain the file and its base; it may not supply
+`BaseDir`. Byte, reader, and mapping inputs with native context provide both `BaseDir` and
+`Root` as slash-relative `fs.ValidPath` directories in `FS` (`.` means its root),
+with the base inside the root. Context paths are checked even when imports and
+overlays are inactive. Diagnostic labels, including URI-looking labels, never
+imply filesystem paths. Nonregular targets are rejected before opening to avoid blocking on FIFOs;
 post-open regular-file checks remain in place under the stable-input assumption.
 ConfigKit canonicalizes symlinks,
 checks containment with relative paths, and opens the canonical root-relative target
 through a library-owned root handle. Contained relative or absolute symlink targets
 are accepted; escapes are rejected. Canonical paths define identity, so hard links at
-distinct paths remain distinct sources.
+distinct paths remain distinct sources. Native filesystems use slash-path operations
+and their own symlink behavior; imports stay in their selected namespace and root.
 
 An active-stack repeat is `import.cycle` and reports the complete cycle's import
 sites. A previously visited canonical path is `import.repeated` and reports both

docs/consumers/README.md

@@ -7,7 +7,7 @@ validation, schema-version policy, CLI parsing, runtime matching and conversion,
 resource checks, remote acquisition, and service initialization.
 
 The maintained [`examples/consumers`](../../examples/consumers) package demonstrates
-four sanitized shapes using only ConfigKit and the Go standard library:
+sanitized shapes using only ConfigKit and the Go standard library:
 
 | Shape | Demonstrated fit | Application code deliberately retained |
 | --- | --- | --- |
@@ -15,6 +15,10 @@ four sanitized shapes using only ConfigKit and the Go standard library:
 | Notarius-like | Canonical object bindings, a string-backed schema version accepting string or integer YAML, strict dynamic options, and `int64` dynamic integers | Catalog selection, allowed option keys, range-checked conversion to runtime `int` |
 | WeatherReporter-like | Defaults-only loading, report maps, canonical module objects, typed option unions, explicit CLI records, and typed secret startup | Report/module compatibility, rejection of explicitly empty domain lists, CLI parsing, and startup ordering |
 | Seriatim-like | Independent strict ordered speaker and autocorrect rule files | First-match and ordered replacement algorithms; no central application configuration file |
+| Bundled file | Simple native `fs.FS` source through ordinary `Load` | Selection and lifetime of the application-owned embedded filesystem |
+| Layered catalog | Embedded defaults with an import, a mapping underlay, an independent host main file, explicit false/zero input, and source-relative resource access | Domain validation and opening/interpreting the template resource |
+| Named presets | One-load sibling inheritance, explicit false, nested destination resolution, and source/derivation inspection | Choosing and executing a preset |
+| Effective configuration diff | Same-type frozen effective comparisons for two selected files or base-versus-overlay impact | Source/options selection and consumer-owned safe presentation |
 
 The YAML under [`testdata`](../../examples/consumers/testdata) is loaded by the
 executable examples and ordinary tests. It contains no private application data,
@@ -48,6 +52,62 @@ both without redefining their syntax. ConfigKit overlays are configuration overl
 they do not select or replace an application's LLM, model, pipeline, report, or
 execution profile concepts.
 
+## Effective configuration diffs
+
+[`CompareFiles`](../../examples/consumers/diff.go) loads two caller-selected files
+with one application definition, handles each load error, calls `configkit.Diff`,
+and presents the safe records through consumer-owned `FormatDifferences`.
+[`CompareRolloutOverlay`](../../examples/consumers/diff.go) loads the same base
+once without an overlay and once with the selected overlay, so omitted lower
+priority values remain present. Its resolver records a derived endpoint URL after
+overlay application, and its nested typed secret reference is classified before
+the resulting records are presented.
+
+The maintained [behavior tests and executable examples](../../examples/consumers/diff_test.go)
+use temporary files and disposable secret values. Presenters should use
+`ValueSnapshot.Present` for absence, record membership for change, and
+`HiddenChanged` for a withheld change indicator. Do not parse the redaction marker
+or add separately rendered full contexts that lack combined sensitivity. The
+[configuration contract](../config.md#effective-configuration-diffs) owns shared
+equality, granularity, and disclosure semantics.
+
+## Named sibling inheritance
+
+[`LoadPresets`](../../examples/consumers/inheritance.go) loads the maintained
+[`presets.yml`](../../examples/consumers/testdata/inheritance/presets.yml) in one
+call. `custom` inherits `enabled: false` from `base`, while its nested
+`settings.primary` changes the inherited `settings.secondary` result at the
+custom destination. The application struct has no `extends` field; ConfigKit
+consumes the relationship before strict typed decoding.
+
+The [executable example](../../examples/consumers/consumers_test.go) reads
+`Metadata.IsExplicit` for the inherited false value, then follows
+`Origin.Inheritance` back to the parent path while `Origin.Locations` identifies
+the declaring source. Applications still choose which preset to execute and
+validate domain meaning. The [configuration contract](../config.md#sibling-inheritance)
+owns exact lookup, precedence, cancellation, and migration rules.
+
+## Native filesystem and layered input
+
+[`LoadBundled`](../../examples/consumers/layered.go) keeps the simple `Load` shape:
+it selects a file in an application-owned embedded filesystem and needs no
+underlay setup. [`LoadCatalog`](../../examples/consumers/layered.go) uses
+`LoadLayered` with an imported embedded-defaults group, a separate
+`Mapping` group, and a host main file supplied by its caller. The groups
+compose strictly within themselves and recursively across precedence boundaries;
+the main file's explicit `false` and `0` replace lower values without being
+mistaken for omissions. Its validator owns allowed modes and resource-name policy.
+
+[`ReadWelcome`](../../examples/consumers/layered.go) follows the effective field's
+origin location to a source ID, queries `Result.SourceContext`, and opens the
+resource relative to that context's `BaseDir`. It does not assume the main file
+owns a retained underlay value. A composite origin needs an application choice;
+ConfigKit does not guess a directory. The context query is passive, while later
+resource reads observe the caller-owned filesystem or a freshly opened host root
+and may fail independently of the completed configuration load. The
+[configuration contract](../config.md#source-resource-contexts) owns exact
+lifetime and precedence rules.
+
 Known module catalogs should use canonical object entries with a module identifier
 and typed options. A small union struct is sufficient when the available option
 fields are known; validation selects which fields apply to each module. Runtime-
@@ -126,6 +186,9 @@ focused behavioral owner:
 | Source/import/selected-overlay pipeline rules and artifact defaults | Narratio-shaped executable example and fixture |
 | Canonical module objects and dynamic option handling | WeatherReporter- and Notarius-shaped examples and migration tests |
 | `LoadSecretsDir`, bootstrap reporting, typed lookup, CLI explanation | `Example_weatherStartup` |
+| Native filesystem, grouped underlays, programmatic mapping, retained resource context | `Example_nativeFilesystem`, `Example_layeredCatalog`, and their focused consumer tests |
+| Named sibling inheritance, presence, and derivation | `Example_inheritedPresets` and `TestPresetCatalogOneLoadAndDerivation` |
+| Effective configuration diffs and consumer-owned presentation | `ExampleDiff`, `ExampleCompareFiles`, and `ExampleCompareRolloutOverlay` |
 | Results, metadata, diagnostics, effective output, and writer failures | Focused root-package load, provenance, rendering, and explanation tests |
 
 The package-level import-boundary test covers the consumer examples too: supported

docs/development.md

@@ -8,22 +8,43 @@ public behavior.
 ## Initial orientation
 
 1. Inspect the working tree and preserve unrelated changes.
-   Agents must read and follow the [agent policy](policy/agents.md) before
-   recommending, planning, or implementing changes.
-2. Read the architecture policy for code or design work.
-3. Read the policy, contract, and internal documents applicable to the task.
+2. Use the Policy Reading Guide below to select applicable policies.
+3. Read the contract and internal documents applicable to the task.
 4. Inspect the relevant implementation and behavioral tests before changing them.
 
+## Policy Reading Guide
+
+| Policy | Read before… |
+| --- | --- |
+| [Policy set provenance](policy/meta.md) | Adopting policies, checking upstream revisions, or updating the adopted policy set. |
+| [Agents](policy/agents.md) | Recommending, planning, or implementing repository changes. |
+| [Architecture](policy/architecture.md) | Planning or changing Go library code or its structure. |
+| [Testing](policy/testing.md) | Selecting validation for code changes, or adding, changing, removing, or evaluating tests. |
+| [Documentation](policy/documentation.md) | Writing, revising, or reorganizing documentation. |
+| [OSS](policy/oss.md) | Adding or updating dependencies or incorporated third-party material, or changing licensing or attribution. |
+| [Release](policy/release.md) | Preparing or publishing a release, changing release procedures, or making versioning and compatibility decisions. |
+
+- Read applicable policies before making decisions they govern; multiple policies
+  may apply.
+- Select policies using the task triggers above. Follow cross-references to
+  relevant sections when needed to resolve a concrete question.
+- Reuse policies already available in context; reread when changed or no longer
+  available.
+- Expand reading when the task's scope expands.
+- When revising a policy itself, read that policy and any provisions elsewhere
+  affected by the revision.
+
 ## Task-specific reading guide
 
 | Task | Read |
 | --- | --- |
-| Repository orientation or component responsibility | [Architecture policy](policy/architecture.md) and [component overview](internal/overview.md) |
-| Documentation changes | [Documentation policy](policy/documentation.md) and the owning document |
-| Architecture decisions or future work | [Documentation policy](policy/documentation.md), [deferred scope](roadmap/deferred.md), and relevant ADRs listed below |
-| Code and test changes | [Architecture policy](policy/architecture.md), [testing policy](policy/testing.md), and the owning contract and tests from the map below |
+| Repository orientation or component responsibility | [Component overview](internal/overview.md) |
+| Documentation changes | The owning document |
+| Licensing, dependencies, attribution, or release-document changes | [Project grant](../COPYRIGHT) and [attribution](oss/attribution.md); release changes also follow the release procedure |
+| Architecture decisions or future work | [Deferred scope](roadmap/deferred.md) and relevant ADRs listed below |
+| Code and test changes | The owning contract and tests from the map below |
 | Examples or copyable assets | The owning implemented contract and [consumer adoption guide](consumers/README.md) |
-| Release preparation | [Release policy](policy/release.md) and [release procedure](release.md) |
+| Release preparation | [Release procedure](release.md) |
 
 Accepted decisions:
 
@@ -33,6 +54,7 @@ Accepted decisions:
 - [Secret references and explicit bootstrap](adr/0004-secret-references-and-explicit-bootstrap.md)
 - [Explicit overrides and safe explanation](adr/0005-explicit-overrides-and-safe-explanation.md)
 - [Keep the public facade root-owned](adr/0006-public-facade.md)
+- [Compare frozen effective configuration snapshots](adr/0010-effective-configuration-diff.md)
 
 For cross-cutting changes, follow every applicable row. Completed implementation
 and remediation plans are preserved in Git history at `cb5b435`; use current
@@ -43,7 +65,9 @@ git show cb5b435:docs/roadmap/remediation.md
 ```
 
 Release-candidate, publication, and verification work follows the operational
-[release procedure](release.md).
+[release procedure](release.md). Its module-selected ReleaseKit `publish`
+invocation validates the committed candidate and publishes only the intended
+tag; those release checks remain distinct from the everyday development check.
 
 The completed string-and-integer schema-version implementation and remediation
 records are preserved in Git history at `244104f`. The
@@ -58,14 +82,40 @@ and current facade ownership belongs to the [component overview](internal/overvi
 
 ## Baseline validation
 
+The completed filesystem-source and underlay roadmap, original feature request,
+implementation plan, and resolved remediation record are preserved in Git history
+at `b604aa1` under `docs/roadmap/`. Current behavior belongs to the
+[configuration contract](config.md#layered-loading). Built-in sibling
+inheritance is now part of the [configuration contract](config.md#sibling-inheritance).
+The completed request, roadmap, plan, and resolved review findings are preserved
+in Git history at `83c59ea` under `docs/roadmap/`.
+
 The completed overlay rename plan and release-workflow simplification record are
 preserved in Git history at `73ce6e9` under `docs/roadmap/`. Current overlay
 behavior belongs to the [configuration contract](config.md#selected-overlays),
 and release operations belong to the [release procedure](release.md).
 
-Install Go 1.25.0 or a supported patched/newer Go toolchain. Acquire declared
-dependencies explicitly before entering an offline development or validation
-environment:
+The completed ReleaseKit adoption, startup performance request and experiments,
+benchmark evidence, performance audit, implementation plan, and remediation
+record are preserved in Git history at `8e91a2b` under `docs/roadmap/`. For example:
+
+```sh
+git show 8e91a2b:docs/roadmap/startup-benchmark.md
+```
+
+Remaining optimization candidates belong to
+[deferred scope](roadmap/deferred.md#deferred-performance-experiments).
+
+The completed effective-configuration diff roadmap, implementation plan, and
+resolved review findings are preserved in Git history at `1c2610c` under
+`docs/roadmap/`. The
+[configuration contract](config.md#effective-configuration-diffs) owns current
+comparison and disclosure behavior, and
+[ADR 0010](adr/0010-effective-configuration-diff.md) records the decision.
+
+Install the Go version required by `go.mod` or a compatible newer toolchain.
+Acquire declared dependencies explicitly before entering an offline
+development or validation environment:
 
 ```sh
 go mod download
@@ -95,10 +145,10 @@ Use focused checks while iterating, with validation proportionate to the change
 under the testing policy. Documentation-only work normally needs the local-link
 check below and whitespace review; behavior-bearing examples and commands also
 need execution. Use `go test -race ./...` for ownership or concurrency changes.
-The [release checker](release.md#4-validate-the-source-candidate) performs one
-minimum-toolchain race-suite run, vet, module hygiene, and Linux cross-builds.
-It already covers examples, links, and package boundaries; do not additionally
-run the everyday checker just to repeat those checks. CI never gates a release.
+The [release procedure](release.md#4-publish-the-committed-candidate) performs
+its selected validation through one `publish` invocation. The everyday checker
+remains a development gate and is not a separate release prerequisite. CI never
+gates a release.
 
 ## Task-to-test map
 
@@ -110,14 +160,22 @@ code changes. Use the component overview to find the corresponding implementatio
 | --- | --- | --- |
 | YAML syntax, diagnostics, parser limits | [Adapter tests](../internal/yaml/parse_test.go), [source translation](../internal/engine/parse_test.go) | `go test ./internal/yaml` and `go test ./internal/engine -run '^TestParseSource'` |
 | File/reader access, imports, globs, containment | [Sources](../internal/engine/source_test.go), [FIFO](../internal/engine/source_fifo_test.go), [imports](../internal/engine/imports_test.go), [globs](../internal/engine/glob_test.go) | `go test ./internal/engine -run 'Source\|Import\|Glob\|Filesystem\|Nonregular\|RegularFile'` |
+| Native filesystem and mapping sources | [Native acquisition](../internal/engine/native_filesystem_test.go), [filesystem facade](../facade_filesystem_test.go), [mapping facade](../facade_mapping_test.go) | `go test ./internal/engine -run 'NativeFilesystem'` and `go test . -run 'TestFacade'` |
 | Composition, selected overlays, CLI precedence | [Composition](../internal/engine/compose_test.go), [overlay](../internal/engine/overlay_test.go), [selected overlays](../internal/engine/selected_overlay_test.go), [overrides](../internal/engine/override_test.go) | `go test ./internal/engine -run 'Compose\|Overlay\|Override'` |
+| Sibling inheritance, provenance, history, sensitivity | [Expansion](../internal/engine/inheritance_test.go), [destination scopes](../internal/engine/inheritance_walk_test.go), [origins](../internal/engine/inheritance_origin_test.go), [history](../internal/engine/inheritance_history_test.go), [secrets](../internal/engine/inheritance_secret_test.go), [public loading](../facade_inheritance_test.go) | `go test ./internal/engine -run 'Inheritance\|Inherited'` and `go test . -run '^TestFacadeInheritance\|^TestFacadeInheritedSecrets'` |
+| Layered loading and source equivalence | [Grouped lifecycle](../internal/engine/load_request_test.go), [layered facade](../facade_layered_test.go), [equivalence](../facade_equivalence_test.go) | `go test ./internal/engine -run '^TestGrouped'`, `go test . -run '^TestFacadeLayered'`, and `go test . -run '^TestEquivalentSourceForms'` |
 | Schema, defaults, projection, cloning | [Schema](../internal/engine/schema_test.go), [decoding](../internal/engine/decode_test.go), [projection](../internal/engine/projection_test.go) | `go test ./internal/engine -run 'Schema\|Decode\|Projection\|Clone\|Inline\|CompareDocument'` |
 | Lifecycle, resolution, validation, callback ownership | [Loading](../internal/engine/load_test.go), [resolution](../internal/engine/resolution_test.go), [validation](../internal/engine/validation_test.go), [ownership](../internal/engine/callback_ownership_test.go), [independence](../internal/engine/independence_test.go) | `go test ./internal/engine -run 'Load\|Resolution\|Validation\|Validator\|Hook\|Callback\|Parallel'` |
 | Paths, metadata, provenance | [Paths](../internal/engine/path_test.go), [metadata](../internal/engine/metadata_test.go), [provenance](../internal/engine/provenance_test.go) | `go test ./internal/engine -run 'Path\|Metadata\|Provenance\|Queries'` |
+| Passive resource contexts | [Engine contexts](../internal/engine/resource_context_test.go), [public contexts](../facade_resource_context_test.go) | `go test ./internal/engine -run 'Resource'` and `go test . -run 'SourceContext'` |
 | Secrets, bootstrap, redaction, explanation | [References](../internal/engine/secret_reference_test.go), [bootstrap](../internal/engine/secrets_test.go), [rendering](../internal/engine/render_test.go), [explanation](../internal/engine/explain_test.go) | `go test ./internal/engine -run 'Secret\|Bootstrap\|Effective\|Explain\|Explanation\|History\|Sensitivity'` |
-| Consumer examples and migration fixtures | [Consumer tests](../examples/consumers/consumers_test.go), [API examples](../example_test.go) | `go test ./examples/consumers` and `go test . -run '^Example'` |
+| Effective configuration diff | [Engine comparison and masking tests](../internal/engine/diff_test.go), [engine operation tests](../internal/engine/diff_operation_test.go), [root facade tests](../facade_diff_test.go), and [consumer examples](../examples/consumers/diff_test.go) | `go test ./internal/engine -run 'Diff|Explain|History|Secret|Independent'`, `go test . -run 'Diff|FacadeInspection|PublicFacadeBoundary|DependencyImportBoundary'`, `go test ./examples/consumers`, and `go test . -run '^ExampleDiff$'` |
+| Consumer examples and migration fixtures | [Consumer tests](../examples/consumers/consumers_test.go), [native and layered examples](../examples/consumers/layered_test.go), [API examples](../example_test.go) | `go test ./examples/consumers` and `go test . -run '^Example'` |
+| Assembly and layered scaling evidence | [Assembly benchmarks](../internal/engine/assembly_bench_test.go), [layered benchmarks](../internal/engine/underlay_bench_test.go) | `go test ./internal/engine -run '^$' -bench '^BenchmarkUnderlay' -benchmem`; repeat with `^BenchmarkCompose` and `^BenchmarkOverlay` |
+| Inheritance scaling evidence | [Expansion benchmarks](../internal/engine/inheritance_bench_test.go) | `go test ./internal/engine -run '^$' -bench '^BenchmarkInheritance' -benchmem` |
 | Documentation links and dependency isolation | [Link checks](../documentation_test.go), [import boundary](../import_boundary_test.go), [public boundary](../public_boundary_test.go) | `go test . -run 'TestLocalDocumentationLinks\|TestDependencyImportBoundary\|TestPublicFacadeBoundary'` |
-| Check-command behavior | [Shell regression](../release_check_test.go) | `go test . -run '^TestCheckScripts'` |
+| Development check behavior | [Development checker test](../development_check_test.go) | `go test . -run '^TestDevelopmentCheckHandlesFilenamesAndStopsOnFailure$'` |
+| Release profile wiring | [Release profile test](../releasekit_profile_test.go) | `go test . -run '^TestReleaseKitProfile$'` |
 
 ## Fuzz validation
 

docs/internal/overview.md

@@ -7,23 +7,32 @@ private state; public signatures and fields do not expose engine types.
 
 Implemented component owners:
 
-- `internal/engine` owns source acquisition, composition, schema inspection,
-  decoding, lifecycle coordination, callback auditing, provenance/history,
-  secret resolution and bootstrap, and safe inspection rendering. It never
-  imports the root package.
-- Root `Source`, `Path`, `Secret`, records, lifecycle callbacks, results, and
-  bootstrap reports are root-defined wrappers or records. Callback wrappers pass
-  only live engine context during a callback; `Result.Config` is detached and its
-  private inspection remains frozen independently.
+- `internal/engine` owns host and native-filesystem source acquisition,
+  grouped strict composition, recursive precedence assembly, and destination-local
+  sibling inheritance before typed decoding. It also owns schema
+  inspection, lifecycle coordination, callback auditing,
+  provenance/history, passive source-context retention, secret resolution and
+  bootstrap, safe inspection rendering, and effective-value comparison over
+  frozen inspection snapshots. It never imports the root package.
+- Root `Source`, `Filesystem`, `Input`, `Underlay`, `Path`, `Secret`, records,
+  lifecycle callbacks, results, and bootstrap reports are root-defined wrappers
+  or records. Callback wrappers pass only live engine context during a callback;
+  `Result.Config` is detached and its private inspection remains frozen
+  independently.
 - Root conversion files explicitly cross the path, record, callback, secret, and
   error boundaries. There are no public aliases, anonymous engine embeddings, or
   public engine bridge methods.
+- Root `Diff` and `Difference` provide the public comparison boundary and
+  convert detached engine records to root-owned paths and snapshots; consumer
+  examples own presentation of the safe changed regions.
 - `internal/document` owns the parser-independent ordered tree and cloning.
   `internal/yaml` is the sole upstream YAML adapter and parser/dumper boundary.
-- Engine mechanism tests own source, composition, decoding, callback, provenance,
-  secret, history, rendering, and bootstrap behavior. Root tests own public API
-  compatibility, facade integration, documentation links, and dependency/public
-  boundary checks. Consumer examples remain under `examples/consumers`.
+- Engine mechanism tests own source, composition, inheritance, decoding,
+  callback, provenance,
+  source-context, secret, history, rendering, and bootstrap behavior. Root tests
+  own public API compatibility, facade integration, documentation links, and
+  dependency/public boundary checks. Consumer examples remain under
+  `examples/consumers`.
 
 Dependency direction is root to engine to document/YAML support. The root may
 import only `internal/engine` among project internal production packages; engine
@@ -32,8 +41,12 @@ these constraints and rejects exported aliases, fields, or signatures involving
 internal types.
 
 The [configuration contract](../config.md) owns implemented YAML and lifecycle
-semantics. The [consumer adoption guide](../consumers/README.md) owns application
-guidance. The [architecture policy](../policy/architecture.md) owns contributor
-boundaries, and [ADR 0006](../adr/0006-public-facade.md) records the facade
+semantics, including [effective configuration diffs](../config.md#effective-configuration-diffs).
+The [consumer adoption guide](../consumers/README.md) owns application guidance.
+The [architecture policy](../policy/architecture.md) owns contributor
+boundaries; [ADR 0006](../adr/0006-public-facade.md) records the facade decision
+and [ADR 0010](../adr/0010-effective-configuration-diff.md) records the diff
 decision. See the [development guide](../development.md) for validation commands
-and [release procedure](../release.md) for source-candidate gates.
+and [release procedure](../release.md) for source-candidate gates. [ADR
+0009](../adr/0009-sibling-inheritance.md) records why inheritance is a built-in
+post-assembly structural operation.

docs/oss/attribution.md

@@ -0,0 +1,43 @@
+# Open Source Attribution Notices
+
+## Standard Repository Policies
+
+Material: `docs/policy/release.md`, adapted from
+`e833faddf120c789a9b45348edaf75720de16348`; `docs/policy/meta.md`, `docs/policy/oss.md`,
+`docs/policy/documentation.md`, `docs/policy/agents.md`,
+`docs/policy/architecture.md`, `docs/policy/testing.md`; root `AGENTS.md`; and the
+Policy Reading Guide in `docs/development.md`, adapted from
+`d84a38ebab26f048cafbe59a2246700c35d29843` for this repository.
+
+Copyright (C) 2026 Eric Rakestraw
+
+SPDX-License-Identifier: CC-BY-SA-4.0
+
+License and notices: [CC BY-SA 4.0](licenses/CC-BY-SA-4.0.txt) |
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/COPYRIGHT) |
+[LICENSE](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/LICENSE) |
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348/COPYRIGHT) |
+[LICENSE](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348/LICENSE).
+
+[Source](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843).
+
+[Source](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348).
+
+## go-yaml v4
+
+Selected module: `go.yaml.in/yaml/v4` at `v4.0.0-rc.6`.
+
+Copyright 2011-2019 Canonical Ltd
+
+Copyright 2025 The go-yaml Project Contributors
+
+Copyright 2006-2010 Kirill Simonov
+
+SPDX-License-Identifier: Apache-2.0 AND MIT
+
+License and notices: [Apache 2.0](licenses/Apache-2.0.txt) |
+[MIT](licenses/MIT.txt) |
+[NOTICE](https://github.com/yaml/go-yaml/blob/1c17b9cee72bf81ae73a8be50fbd7dd3a9985125/NOTICE) |
+[LICENSE](https://github.com/yaml/go-yaml/blob/1c17b9cee72bf81ae73a8be50fbd7dd3a9985125/LICENSE).
+
+[Source](https://github.com/yaml/go-yaml/tree/1c17b9cee72bf81ae73a8be50fbd7dd3a9985125).

docs/oss/licenses/Apache-2.0.txt

@@ -0,0 +1,202 @@
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docs/oss/licenses/CC-BY-SA-4.0.txt

@@ -0,0 +1,170 @@
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+     c.	For the avoidance of doubt, this Section 6(b) does not affect any right the Licensor may have to seek remedies for Your violations of this Public License.
+
+     d.	For the avoidance of doubt, the Licensor may also offer the Licensed Material under separate terms or conditions or stop distributing the Licensed Material at any time; however, doing so will not terminate this Public License.
+
+     e.	Sections 1, 5, 6, 7, and 8 survive termination of this Public License.
+
+Section 7 – Other Terms and Conditions.
+
+     a.	The Licensor shall not be bound by any additional or different terms or conditions communicated by You unless expressly agreed.
+
+     b.	Any arrangements, understandings, or agreements regarding the Licensed Material not stated herein are separate from and independent of the terms and conditions of this Public License.
+
+Section 8 – Interpretation.
+
+     a.	For the avoidance of doubt, this Public License does not, and shall not be interpreted to, reduce, limit, restrict, or impose conditions on any use of the Licensed Material that could lawfully be made without permission under this Public License.
+
+     b.	To the extent possible, if any provision of this Public License is deemed unenforceable, it shall be automatically reformed to the minimum extent necessary to make it enforceable. If the provision cannot be reformed, it shall be severed from this Public License without affecting the enforceability of the remaining terms and conditions.
+
+     c.	No term or condition of this Public License will be waived and no failure to comply consented to unless expressly agreed to by the Licensor.
+
+     d.	Nothing in this Public License constitutes or may be interpreted as a limitation upon, or waiver of, any privileges and immunities that apply to the Licensor or You, including from the legal processes of any jurisdiction or authority.
+
+Creative Commons is not a party to its public licenses. Notwithstanding, Creative Commons may elect to apply one of its public licenses to material it publishes and in those instances will be considered the “Licensor.” Except for the limited purpose of indicating that material is shared under a Creative Commons public license or as otherwise permitted by the Creative Commons policies published at creativecommons.org/policies, Creative Commons does not authorize the use of the trademark “Creative Commons” or any other trademark or logo of Creative Commons without its prior written consent including, without limitation, in connection with any unauthorized modifications to any of its public licenses or any other arrangements, understandings, or agreements concerning use of licensed material. For the avoidance of doubt, this paragraph does not form part of the public licenses.
+
+Creative Commons may be contacted at creativecommons.org.

docs/oss/licenses/MIT.txt

@@ -0,0 +1,18 @@
+MIT License
+
+Copyright (c) <year> <copyright holders>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and
+associated documentation files (the "Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the
+following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial
+portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT
+LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO
+EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+USE OR OTHER DEALINGS IN THE SOFTWARE.

docs/policy/agents.md

@@ -1,12 +1,14 @@
 # Agent Policy
 
-**Base Policy Version:** 1.1
+**Base Policy Version:** 1.3
 
-**Last Updated**: 2026-09-23
+**Last Updated**: 2026-09-28
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/agents.md)
 
 ## Purpose
 
-Agents working in this repository must read and adhere to the following house rules.
+Agents must read and follow these rules.
 
 ## Implementation Discipline
 
@@ -14,8 +16,6 @@ Agents working in this repository must read and adhere to the following house ru
 request. Do not implicitly strengthen requirements, restrictions, or guarantees
 beyond what the user actually requested.
 
-Simplify the mechanism, not the user's choices.
-
 **Keep implementation machinery proportionate.** Use straightforward,
 idiomatic code and introduce only the machinery needed to support the requested
 behavior and flexibility. Optimize for conceptual simplicity and
@@ -57,15 +57,15 @@ the ambiguity rather than treating repetition as approval.
 
 ## Git Commit Policy
 
-By default, create a commit after completing each coherent unit of work, using a
+By default, commit each completed, nonempty unit of work using a
 concise, plain-English message describing what changed. This policy is standing
 authorization to create those local commits; a separate user request is not
 required. An explicit user instruction to leave work uncommitted takes precedence.
 
 A unit of work is a focused, reviewable outcome, including its relevant tests and
-documentation. Do not commit after every intermediate edit or create empty commits.
+documentation.
 Complete the appropriate validation before committing. If work is incomplete or
-validation is blocked, report that state rather than presenting it as completed.
+validation is blocked, report that state accurately.
 
 Before committing:
 
@@ -73,10 +73,10 @@ Before committing:
    changes. Identify changes that predate the current unit of work.
 2. Exclude secrets, local environment files, generated junk, and unrelated changes.
 3. Stage only the files or hunks belonging to the completed unit. Use `git add .`
-   only when every included change belongs to that unit. Do not disturb unrelated
-   staged or unstaged work.
-4. Review `git diff --staged` and run appropriate checks for the change. Do not
-   repeat checks already passed for unchanged content merely to create a commit.
+   only when every included change belongs to that unit. Preserve unrelated
+   staged and unstaged work.
+4. Review `git diff --staged` and run appropriate checks for the change. Reuse
+   successful checks for unchanged content.
 5. Review `git log --oneline -n 10` and use
    `git commit -m "<concise plain-English commit message>"`.
 6. Report the commit identifier and any remaining uncommitted work.

docs/policy/architecture.md

@@ -1,18 +1,55 @@
 # Architecture Policy
 
-**Base Policy Version:** 1.0
+**Base Policy Version:** 1.3
 
-**Last Updated:** 2026-09-06
+**Last Updated**: 2026-09-29
 
-## Purpose and applicability
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/architecture.md)
+
+## Purpose And Ownership
+
+The adopted policy defines the application's purpose and the architectural
+constraints contributors must follow. It owns system boundaries, responsibility
+and dependency rules, established invariants, and explicit non-goals.
+
+Describe the concrete package inventory and implementation mechanics in internal
+documentation. Link to canonical public contracts for their exact behavior and
+to ADRs for decision rationale.
 
 ConfigKit is an opinionated Go library for consistent YAML configuration loading
-and resolution across consuming applications. This policy governs contributions.
-The [component overview](../internal/overview.md) owns implementation inventory, the
-[configuration contract](../config.md) owns implemented behavior, and the
-[deferred scope](../roadmap/deferred.md) preserves deferred scope.
+and resolution across consuming applications. The [component overview](../internal/overview.md)
+owns implementation inventory, the [configuration contract](../config.md) owns
+implemented behavior, and the [deferred scope](../roadmap/deferred.md) preserves
+deferred scope.
 
-## Success criteria
+## Adopted Constraints And Implemented Structure
+
+Architectural constraints govern current and future development. Record
+implemented structure accurately, proposals and implementation status in the
+roadmap, and significant decisions and rationale in ADRs. Describe a target
+structure as implemented when its changes land.
+
+Constraints must reflect deliberately selected requirements and decisions.
+Do not promote incidental implementation choices into architectural invariants.
+
+## Success Criteria
+
+ConfigKit prioritizes a coherent configuration model: consistency and
+predictability take precedence over flexibility for accommodating individual
+downstream conventions. Consumers should learn one set of rules, not different
+combination or presence semantics depending on the source format or consuming
+application. Distinct operations such as strict composition and intentional
+overlay express distinct purposes within that model.
+
+Consumers should be able to load once and receive a complete, validated
+configuration with inspectable provenance for its final values, including
+defaults and recorded application-derived values. Configuration and metadata
+together must distinguish explicit zero, false, empty, and null input from
+omission; ordinary Go values alone need not encode those distinctions.
+Application-owned resolution belongs within the recorded loading lifecycle, so
+consumers need no subsequent generic merging, defaulting, or provenance
+reconstruction. This does not transfer domain execution or runtime resource
+management into ConfigKit.
 
 1. **Reduce application infrastructure:** Can applications delete substantial
    configuration infrastructure while retaining clear ownership of their domain
@@ -21,8 +58,11 @@ The [component overview](../internal/overview.md) owns implementation inventory,
    configuration model, with only the flexibility needed to express genuine
    application requirements?
 
-Both criteria must hold. ConfigKit should not preserve divergent consumer
-conventions merely for compatibility, nor impose restrictions that prevent useful
+Both criteria must hold. Minimize consumer loading and processing code subject
+to the consistency, presence, and provenance principles above, rather than
+trading those principles for consumer-specific shortcuts. ConfigKit should not
+preserve divergent consumer conventions merely for compatibility, nor impose
+restrictions that prevent useful
 domain information from being expressed. Prefer a canonical representation using
 existing primitives before adding another mechanism. Extend the model when a
 concrete requirement cannot be accommodated cleanly, not simply because a
@@ -51,7 +91,7 @@ boundaries, and keep simple consumers independent of optional composition
 features. The [consumer adoption guide](../consumers/README.md) demonstrates the
 implemented surface against representative simple and complex shapes.
 
-## Responsibility boundaries
+## Responsibility Boundaries
 
 ConfigKit owns configuration input processing and the coordination of decoding,
 resolution, validation, and inspection. Applications own their configuration
@@ -68,7 +108,23 @@ must never run implicitly as part of configuration loading.
 Remote data can enter through application-supplied input without introducing a
 network provider into the library.
 
-## Public API and dependency direction
+## Loading Lifecycle
+
+Configuration loading is a synchronous, complete-on-return operation. A successful
+load provides the final configuration and inspection metadata after the configured
+processing and validation phases complete. A failed load reports diagnostics and
+returns no usable partial result. The normal application pattern is one load near
+startup; applications may explicitly initiate additional loads.
+
+ConfigKit releases its library-owned loading handles before returning and remains
+inactive until the application explicitly invokes it again. It performs no
+background watching, refresh, or mutation of published results. Caller-owned
+resources and application callback effects remain the caller's responsibility.
+The [configuration contract](../config.md#completion-and-resource-lifetime) owns
+the precise completion and ownership behavior. The proposed decision record is
+[ADR 0008](../adr/0008-complete-on-return-loading.md).
+
+## Public API And Dependency Direction
 
 Keep one small, coherent public surface for the supported lifecycle. Simple
 single-document consumers must not need import, overlay, or provider machinery.
@@ -94,20 +150,43 @@ Dependencies must preserve those boundaries; package splits and abstractions
 must earn their complexity. Do not impose a full application or hexagonal
 framework on this library.
 
-## Configuration integrity
+## Configuration Integrity
 
-Composition and intentional overlay are distinct operations. Composition must
-not silently become last-wins override through file ordering. The decision and
-alternatives are recorded in [ADR 0001](../adr/0001-separate-composition-and-overlay.md).
+Within a configuration tier, independent contributions use strict composition:
+shared mappings combine recursively, while overlapping non-mapping definitions
+are conflicts, including equal values. Between precedence tiers, intentional
+overlay combines mappings recursively, retains omitted lower-priority children,
+and replaces other present values according to the existing overlay rules.
+Tier precedence is explicit; traversal order within a tier establishes stable
+reporting, not permission to override. The distinction and its rationale are
+recorded in [ADR 0001](../adr/0001-separate-composition-and-overlay.md).
+
+The main document and its imports form a strictly composed tier. Explicit
+underlay groups compose strictly within each group, then combine recursively
+from lowest to highest beneath the main tier. Source acquisition kind does not
+determine its precedence role. Input-specific constraints, including CLI path
+rules and opt-in version ownership, remain part of the configuration contract. Consistent combination
+semantics do not require identical acquisition or lifetime behavior from every
+source kind.
+
+After source precedence is complete, built-in sibling `extends` relationships
+resolve structurally in the same parser-independent representation, using the
+existing recursive overlay rules. This adds neither an acquired source nor a
+consumer registration mechanism. Enclosing relationships precede nested
+destination-local resolution; typed defaults, application resolution, and
+secret lookup retain their existing lifecycle roles. The [configuration
+contract](../config.md#sibling-inheritance) owns the exact syntax and presence
+rules, and [ADR 0009](../adr/0009-sibling-inheritance.md) records the rationale.
 
 ConfigKit must define supported YAML semantics explicitly rather than inherit
 parser defaults. Processing order and diagnostics must be deterministic for the
 same inputs and options. Invalid input must not produce a successful usable
 configuration. Do not run application hooks on invalid typed configuration.
 
-User-controlled precedence is application defaults, composed base, selected
-overlay, then explicitly supplied CLI overrides. Applications own flag parsing;
-ConfigKit owns overlay semantics and provenance. Domain resolution follows these
+User-controlled precedence is application defaults, ordered underlays, composed
+main input, selected overlay, then explicitly supplied CLI overrides.
+Applications own flag parsing; ConfigKit owns overlay semantics and provenance.
+Domain resolution follows these
 inputs and must remain recorded, not become a hidden override channel.
 
 Preserve node-based composition until the effective input is ready for typed
@@ -122,7 +201,14 @@ See [ADR 0005](../adr/0005-explicit-overrides-and-safe-explanation.md).
 Detailed syntax and operation tables belong to the
 [current configuration contract](../config.md).
 
-## Ownership and confidentiality
+The frozen inspection boundary also supports comparing completed same-type
+results without reopening sources or rerunning the load lifecycle. The
+[configuration contract](../config.md#effective-configuration-diffs) owns
+comparison and disclosure behavior, the [component overview](../internal/overview.md)
+owns implementation responsibility, and [ADR 0010](../adr/0010-effective-configuration-diff.md)
+records the accepted decision.
+
+## Ownership And Confidentiality
 
 Returned configuration and metadata must not share mutable state across loads.
 ConfigKit must not mutate a result after returning it. Ordinary exported Go
@@ -143,7 +229,7 @@ forwarded blindly when they can include raw input. Do not log configuration
 implicitly. Applications remain responsible for classifying their secrets and
 for safe messages supplied by their hooks.
 
-## Scope boundaries
+## Scope Boundaries
 
 Do not introduce automatic directory discovery, network providers, arbitrary
 merge strategies, additional configuration languages, or application runtime

docs/policy/documentation.md

@@ -1,126 +1,176 @@
 # Documentation Policy
 
-**Base Policy Version:** 1.1
+**Base Policy Version:** 1.11
 
-**Last Updated:** 2026-09-23
+**Last Updated**: 2026-09-28
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/documentation.md)
 
 ## Purpose
 
-Assign each documentation topic one canonical owner so ConfigKit's documentation
-remains accurate and useful to consumers, maintainers, and implementation agents.
+This policy assigns each documentation topic to one canonical owner. Its goal is
+to keep ConfigKit's documentation accurate, concise, discoverable, and resistant
+to drift for consumers, maintainers, developers, and coding agents.
 
-## Core rules
+## Core Rules
 
-### One canonical owner
+### One Canonical Owner
 
-Each authoritative fact belongs in one owning document or Go declaration's
-reference documentation. Other documents may summarize and link to that owner;
-they must not maintain competing definitions of commands, API contracts, YAML
-semantics, defaults, paths, diagnostics, or compatibility guarantees.
+Each authoritative fact belongs in one document. A non-owning document may give
+a short, stable summary for orientation, but it must link to the canonical owner
+for volatile details.
 
-### Policies, current behavior, and future work
+Volatile details include commands, flags, configuration fields and defaults,
+module keys, schemas, file names, paths, status codes, retry behavior, and
+runtime guarantees. If readers could reasonably treat a statement as a
+contract, maintain it only in the owning document.
 
-- Policies state currently binding contributor rules, including constraints on
-  future implementation. They do not imply that features already exist.
-- Current API/configuration contracts, consumer guidance, and component
-  documentation describe implemented behavior only, including partial boundaries.
-- ADRs record accepted decisions and rationale. Acceptance is not evidence of
-  implementation; execution status belongs in roadmaps.
-- Roadmaps own unimplemented targets, deferred work, and execution status.
+### Current And Future Behavior
 
-Future implementation plans own their task order, acceptance gates, and progress;
-`docs/roadmap/deferred.md` owns deliberate exclusions, not execution instructions.
-As behavior lands, move its normative definition into the current contract and
-replace the roadmap definition with a link. Retire completed roadmap material
-as part of release preparation; ADRs preserve significant decision history.
+Outside `docs/roadmap/`, documentation describes implemented behavior only.
+Partial features may be described only to their implemented boundary.
 
-### Audience and examples
+Architectural constraints may govern future changes. Describe current structure
+from implemented behavior.
 
-Consumer documentation explains how to use the library and what consumers own.
-Internal documents explain mechanisms and dependencies. Keep implementation
-mechanics out of consumer guidance unless they affect a meaningful choice.
+ADRs may record an accepted architectural decision before implementation. The
+roadmap owns implementation status and sequencing until the decision is
+implemented. Current architecture, user, operator, integration, and internal
+documentation are updated when the behavior lands.
 
-Complete copyable artifacts belong in `examples/`; Go API examples may live in
-executable Go example tests beside the relevant package. Prose should use the
-smallest useful excerpt and link to the maintained artifact. Examples must be
-secret-free and checked against implemented behavior. Clearly label illustrative
-future syntax in roadmaps; do not present it as a runnable current example.
+`docs/roadmap/deferred.md` owns deliberate exclusions; task-specific plans
+own execution order and progress. Completed plans retire during release
+preparation.
 
-### Security and privacy
+### Audience And Detail
 
-Do not include real credentials, private keys, environment dumps, sensitive input,
-or private infrastructure details unless intentionally public. Document secret
-handling, not secret values.
+Write for the document's stated audience and include only the detail needed for
+its owned topic. Keep user and operator docs focused on using and operating the
+application. Developer docs should link to user-facing and external contracts
+for their definitions.
 
-## Canonical ownership
+### Examples
 
-| Topic | Owner | Boundary |
-| --- | --- | --- |
-| Product orientation | `README.md` | Current status, purpose, navigation, and a minimal working example |
-| Contributor entry point | `docs/development.md` | Reading guide and actual baseline validation commands |
-| Architecture rules | `docs/policy/architecture.md` | Responsibilities, dependency constraints, invariants, and scope boundaries |
-| Agent rules | `docs/policy/agents.md` | Implementation discipline, proportionality, rules against inventing contracts, and local commit authorization; not application contracts or architecture |
-| Documentation rules | `docs/policy/documentation.md` | Ownership and document lifecycle |
-| Testing rules | `docs/policy/testing.md` | Risk-based sufficiency, test boundaries, and maintenance principles |
-| Release rules | `docs/policy/release.md` | Compatibility, integrity, gates, immutability, and exceptions |
-| Release procedure | `docs/release.md` | Exact maintainer commands, prerequisites, supported targets, and verification |
-| Exported Go API | Go documentation beside declarations | Symbol contracts, errors, ownership, and lifecycle API behavior |
-| YAML and resolution semantics | `docs/config.md` | Syntax, composition, precedence, conversion, secret references/bootstrap, and explanation semantics; Go reference docs link here for shared rules |
-| Consumer walkthroughs | `docs/consumers/`, when useful | Task-oriented usage and consumer responsibilities, not duplicate API reference |
-| Component inventory | `docs/internal/overview.md` | Implemented components and links to their owners |
-| Component mechanics | Other `docs/internal/` files, when needed | Detailed implementation behavior and change recipes |
-| Architectural decisions | `docs/adr/` | Context, decisions, alternatives, consequences, and supersession history |
-| Deferred scope | `docs/roadmap/deferred.md` | Deliberate exclusions and the conditions for reconsidering them |
-| Future implementation plans | Task-specific files under `docs/roadmap/` | Unfinished work, task order, acceptance gates, and progress; completed plans retire to Git history |
-| Historical release notes | `docs/releases/` | Version-specific historical summaries required by release policy |
-| Complete examples | `examples/` and executable Go examples | Maintained artifacts; prose links instead of duplicating full examples |
+Complete copyable files belong in `examples/`. Documentation may use the
+smallest illustrative snippet needed to explain its owned topic, but should link
+to maintained examples for complete copyable files.
 
-A listed owner is not a requirement to create an empty file before it has useful
-content. Create API, consumer, example, and component documents when the corresponding behavior
-exists and there is useful content to own. Link only to existing files; use plain
-code paths for planned files. Unvalidated candidate and publication work has the
-explicit temporary exception recorded in the release policy.
+Executable Go API examples may live beside the relevant package.
 
-ConfigKit has no owned HTTP service, application CLI, or runtime operations
-interface. Do not create those reference documents for consuming applications.
+Examples must be valid, secret-free, and tested where practical. Commands and
+configuration used in documentation should match the application.
 
-## Boundary rules
+### Security And Privacy
 
-Architecture owns governing constraints; ADRs explain why; contracts describe
-implemented semantics; internal documentation explains how. A roadmap may link
-to all of them but must not become a second current reference.
+Documentation and examples must not contain real credentials, private keys,
+private environment dumps, sensitive source material, or private infrastructure
+details unless intentionally public. Document secret-handling mechanisms, not
+secret values.
 
-Release policy owns guarantees; the release procedure owns commands. A release
-note cannot redefine a compatibility contract or replace a current-documentation
-update. The note at an immutable tag is that release's historical record.
+## Canonical Ownership
 
-This policy owns what documentation may disclose. Architecture owns security
-invariants. Current configuration and Go API contracts own secret-classification
-and rendering behavior when implemented; internal docs own their mechanisms.
+| Topic | Canonical owner | Owned content | Content owned elsewhere |
+| --- | --- | --- | --- |
+| Product orientation and minimal example | `README.md` | Purpose, current status, shortest useful Go example, and navigation, including licensing and attribution. | Full Go API and configuration contracts, release procedure, and implementation detail. |
+| Contributor entry point | `docs/development.md` | Task-oriented reading guide, contributor orientation, baseline validation commands, and canonical links. | Package inventory, architecture rules, subsystem behavior, detailed change recipes. |
+| Application architecture policy | `docs/policy/architecture.md` | System shape, normative ownership, dependency direction, boundaries, invariants, safety properties, and non-goals. | Concrete package inventory, implementation mechanics, contributor procedures, decision history, future work. |
+| Policy-set provenance and upstream navigation | `docs/policy/meta.md` | Policy-set origin, local authority, adoption baselines, and upstream adoption and update links. | Task-specific reading triggers, detailed adoption procedures, and formal attribution notices. |
+| Agent policy | `docs/policy/agents.md` | Proportionate, idiomatic implementation; rules against inventing requirements, restrictions, guarantees, or policy. | Application contracts, architecture, testing policy, and contributor procedures. |
+| Documentation policy | `docs/policy/documentation.md` | Documentation ownership, audience boundaries, maintenance rules, and ADR/document lifecycle. | Application architecture or product behavior. |
+| Testing policy | `docs/policy/testing.md` | Test philosophy, risk sufficiency, boundaries, doubles, coverage, regressions, and test maintenance. | Subsystem behavior, application contracts, subsystem-specific test inventories, and implementation plans. |
+| Release policy | `docs/policy/release.md` | Versioning, compatibility, release integrity and immutability, release documentation, proportionate gates, publication controls, completion, and required versus optional checks. | Repository-specific commands, product installation guidance, CLI version semantics, historical release summaries, and CI implementation detail. |
+| Open source licensing and attribution policy | `docs/policy/oss.md` | License defaults and author choices, COPYRIGHT and attribution formats, license/notice links, discoverability, maintenance, and concrete exceptions. | Component entries, license texts, release commands, and dependency security review. |
+| Project copyright and license grant | `COPYRIGHT` | Project identity, copyright holders and years, SPDX expression, code grant, separately licensed document scope, warranty disclaimer, and third-party statement. | Complete license texts and upstream component attribution. |
+| Project license texts | `LICENSE` | Complete project and incorporated companion license texts, with required project-specific fields. | General licensing policy and upstream component attribution. |
+| Upstream component attribution | `docs/oss/attribution.md` | House Format entries for dependencies and adapted documents: identity, material scope, copyright, license, notices, and source links. | General policy, exception reasoning, and complete standard license texts. |
+| Standard license texts | `docs/oss/licenses/` | Complete, verbatim standard license texts shared by applicable entries, including copied documentation. | Project-specific upstream notices, attribution prose, and policy interpretation. |
+| Repository-specific release procedure | `docs/release.md` | Maintainer prerequisites, candidate preparation, validation and publication commands and automated guards, asynchronous deliverables, delivery status, optional consumer verification, and recovery. | General release requirements, product installation guidance, historical release summaries, and current compatibility contracts. |
+| Exported Go API | Go documentation beside declarations | Symbol contracts, errors, ownership, and lifecycle behavior. | Shared YAML semantics and complete examples. |
+| Configuration contract | `docs/config.md` | YAML syntax, composition, precedence, schema, defaults, resolution, validation, secrets, and explanation semantics. | Go symbol contracts, complete examples, and implementation detail. |
+| Consumer guidance for the public Go API | `docs/consumers/` | Task-oriented public Go API usage, minimal examples, and consumer responsibilities. | Go symbol contracts, configuration semantics, and internal implementation detail. |
+| Inventory of implemented internal packages | `docs/internal/overview.md` | Implemented packages and components, responsibilities, and links to focused internal docs. | Normative architecture, contributor reading policy, external contracts. |
+| Behavior and architecture of internal packages | Other files under `docs/internal/` | Implementation flow, collaborators, state transitions, package-local guarantees and failures, and relevant tests. | Global architecture invariants, configuration definitions and defaults, external schemas, operator procedures. |
+| Architectural decision records | `docs/adr/` | Significant decisions, context, alternatives, rationale, consequences, and supersession history. | Current behavior reference, implementation status, task sequencing. |
+| Deferred scope | `docs/roadmap/deferred.md` | Deliberate exclusions and conditions for reconsideration. | Task execution and current behavior. |
+| Future implementation plans | Task-specific files under `docs/roadmap/` | Unfinished work, sequencing, acceptance, and progress; completed plans retire during release preparation. | Implemented behavior and ADR rationale. |
+| Release-note history | `docs/releases/` | One checked-in summary per source release under the procedure; the note at its immutable tag is the release record. | Current commands, behavior, contracts, and compatibility definitions. |
+| Complete examples | `examples/` and executable Go examples | Maintained copyable artifacts and Go API examples. | Configuration reference, symbol contracts, and prose explanation. |
 
-## Architectural decision records
+Create supporting documents for existing interfaces and responsibilities when
+substantive content is available.
 
-Use sequential filenames, such as `0001-separate-composition-and-overlay.md`,
-and the lightweight Nygard structure: title, status, date, context, decision,
-alternatives considered, and consequences.
+Do not create empty owners.
 
-Accepted decision content is immutable. A changed decision requires a new ADR
-and a superseded status/link in the old one. Navigation links may be repaired
-when specification ownership moves without rewriting historical rationale.
-Rejected architectural alternatives belong in ADRs; deferred or rejected product
-features belong in roadmaps. Do not create an ADR for every routine choice.
+## Boundary Rules
 
-## Maintenance and verification
+### Orientation
 
-Update the owning current documentation in the same change that implements or
-changes behavior. When ownership moves, remove the old definition and preserve
-useful navigation links.
+Link among orientation, architecture, and internal documents; maintain each
+package or behavior description in its canonical owner.
+
+### Go API And Configuration
+
+Go declaration documentation owns exported symbol behavior. The configuration
+contract owns shared YAML and resolution semantics. Consumer guidance explains
+tasks and links to those contracts.
+
+### Releases
+
+The release policy defines requirements and invariants that every release must
+satisfy. The repository-specific release procedure defines the commands and
+workflow maintainers use to satisfy that policy. Release notes record history
+at immutable tags. Maintain current behavior in its canonical documentation,
+update it with the corresponding behavior changes, and keep release procedures
+consistent with adopted policy.
+
+### Project Licensing And Upstream Attribution
+
+README and release documents link to the canonical licensing and attribution
+materials under the [OSS policy](oss.md). The release procedure owns URL
+construction and publication mechanics.
+
+### Contracts And Implementation
+
+Go API and configuration documents define externally observable behavior.
+Internal documents explain implementation and link to the owning contract.
+
+### Security Topics
+
+This policy owns what documentation and examples may contain. Architecture owns
+application security invariants. The configuration and public API contracts own
+secret-classification, bootstrap, and rendering behavior. Internal docs own
+implementation mechanisms only.
+
+## Architecture Decision Records
+
+Use sequentially numbered ADR filenames such as
+`0001-record-architecture-decisions.md`. Follow the lightweight Nygard format:
+
+1. title;
+2. status;
+3. date;
+4. context;
+5. decision;
+6. alternatives considered;
+7. consequences.
+
+Treat the decision content of an accepted ADR as immutable. When a decision
+changes, create a new ADR and update the earlier ADR's status to superseded.
+Rejected architectural alternatives belong in the ADR; rejected product ideas
+belong in the roadmap. Routine choices need no ADR.
+
+## Maintenance
+
+When behavior changes, update its canonical owner in the same change. If
+ownership moves, remove the old definition and replace it with a link where
+navigation remains useful.
 
 Before completing documentation work:
 
-- verify claims against the implementation or clearly identify future design;
-- check changed local links, paths, fragments, examples, and commands;
-- preserve the distinction between policy, decisions, target behavior, and status;
-- check canonical ownership and remove stale or duplicate definitions;
-- inspect formatting and whitespace;
-- ensure no sensitive data was introduced.
+- verify affected behavior and examples;
+- check commands, flags, fields, defaults, schemas, and paths against their
+  implementation;
+- keep unimplemented behavior in the roadmap, subject to the ADR exception;
+- remove stale references and validate links;
+- confirm that non-owning documents summarize and link rather than redefine;
+- confirm that no secrets or sensitive private data were added.

docs/policy/meta.md

@@ -0,0 +1,27 @@
+# Policy Set Provenance
+
+**Base Policy Version:** 1.0
+
+**Last Updated**: 2026-09-28
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/meta.md)
+
+## Origin And Local Authority
+
+This repository's policy set is adapted from
+[Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy).
+The adopted local policies govern work in this repository. Upstream revisions
+become local policy through deliberate adoption, preserving local decisions.
+Each policy's `Upstream Source` identifies its adopted baseline.
+
+## Adoption And Updates
+
+For policy adoption, upstream revision reviews, and updates, consult the
+[upstream adoption and update guide](https://gitea.maximumdirect.net/eric/policy/src/branch/main/docs/adoption.md).
+That guide and the reference example collection are maintained upstream and
+accessed through links. Keep discovery links pointed at the current upstream
+project and guide; keep adopted-source and example links pinned to commits.
+
+The local [developer guide](../development.md) routes tasks to applicable
+policies. Formal credit and licensing notices for the copied policy material
+belong in the local [attribution document](../oss/attribution.md).

docs/policy/oss.md

@@ -0,0 +1,217 @@
+# Open Source Licensing And Attribution Policy
+
+**Base Policy Version:** 1.8
+
+**Last Updated**: 2026-09-28
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/oss.md)
+
+## ConfigKit Applicability
+
+ConfigKit's selected grant is LGPL-3.0-or-later, recorded in
+[COPYRIGHT](../../COPYRIGHT), with the complete LGPLv3 and incorporated GPLv3
+texts in [LICENSE](../../LICENSE). The [release procedure](../release.md)
+owns source-only Go module publication through Gitea. No separate hosted release
+object or license asset is required for this channel.
+
+## Purpose And Scope
+
+This policy defines project licensing conventions and a repeatable House Format
+for upstream attribution and license notices. It covers the project's own code
+and upstream components incorporated into or distributed with the application
+or library, including relevant transitive dependencies and copied code,
+documentation, or assets.
+Determine treatment from the component's terms and actual use.
+
+Consulting development guidance or using tooling alone does not require an
+attribution entry. Copying or adapting documentation for inclusion in the
+repository is different: preserve its applicable attribution and license terms.
+A material's role as development guidance does not exempt copied documents.
+Apply any express permissions for attribution-free reuse of example formatting.
+
+The baseline is a publicly available source repository with releases published
+alongside it. The format is independent of language, forge, and build system.
+It does not require attribution for every development tool, establish license
+compatibility, or resolve every obligation arising from use or distribution.
+
+## Canonical Locations
+
+The [documentation policy](documentation.md) assigns ownership:
+
+- `COPYRIGHT`: project name, copyright statements, SPDX expression, grant,
+  warranty disclaimer, and third-party statement.
+- `LICENSE` contains the complete, unmodified project license text, including
+  incorporated companion text. Dependency attribution does not relicense the project.
+- `docs/oss/attribution.md`: canonical component entries.
+- `docs/oss/licenses/`: complete, verbatim standard and applicable companion texts.
+
+Use recognizable license filenames, such as `Apache-2.0.txt` or `LGPL-3.0.txt`.
+A shared standard text may serve multiple entries. Keep explanations outside
+license texts and preserve distinct versions and exceptions.
+
+## Upstream Reference Examples
+
+Use the commit-pinned upstream examples to create actual project notices.
+Retain their links and keep the example collection upstream; keep applicable
+license texts locally as specified above. The
+[standard license-text collection](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/oss/licenses/README.md)
+identifies the available reference texts.
+
+## Project Licensing
+
+Unless the author specifies otherwise, use **GPL version 3 or later**
+(`GPL-3.0-or-later`) for applications and **LGPL version 3 or later**
+(`LGPL-3.0-or-later`) for libraries. An author's explicit choice takes precedence:
+for example, a one-off script may use MIT. Such a choice requires no exception
+rationale, additional approval, or compensating controls under this policy.
+
+These defaults guide new license selection. Adopting this policy does not
+relicense existing code or override third-party terms. Preserve an existing
+license unless the author deliberately changes it. For mixed-purpose projects,
+follow the author's selected scope rather than inferring different licenses for
+individual packages.
+
+Use the upstream [application COPYRIGHT example](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/examples/licensing/application/COPYRIGHT)
+or [library COPYRIGHT example](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/examples/licensing/library/COPYRIGHT)
+to create the project’s root `COPYRIGHT` file.
+Use verified project names, contribution years, and copyright holders.
+For another selected license, adapt the grant, SPDX expression, disclaimer, and
+text references together to reflect the author's selected grant.
+
+For GPLv3 projects, copy the complete GPLv3 text into `LICENSE`. For LGPLv3
+projects, place the complete LGPLv3 text followed by the complete GPLv3 text in
+`LICENSE`; the LGPL incorporates the GPL terms. Keep the texts unmodified and
+place project-specific statements in `COPYRIGHT`. For the default grants, the
+grant and SPDX expression in `COPYRIGHT` must expressly identify `or-later`;
+follow the author’s choice if a different grant is selected.
+
+For other licenses, provide their complete text and any required project-specific
+fields; the standard reference collection remains unchanged. Keep `COPYRIGHT`,
+`LICENSE`, SPDX expressions, and README descriptions consistent with the actual
+grant. This convention introduces no blanket requirement for per-file headers;
+preserve notices or add them where the applicable terms require them.
+
+Identify separately licensed documentation or assets in the project grant, with
+links to their attribution and license information. An application's default
+code license does not override the terms of incorporated documentation.
+
+## Upstream Attribution House Format
+
+Begin `docs/oss/attribution.md` with `# Open Source Attribution Notices`,
+followed directly by component entries. These comprise the complete document.
+
+Each entry presents these five fields in order, with blank lines between them:
+
+1. **Project name** as a level-two heading. An optional identification paragraph
+   may follow, such as `Selected module: MODULE at VERSION.` with the module and
+   version in backticks. For copied documentation, a concise `Material:` sentence
+   may identify the affected files and whether they were adapted or copied
+   without modification. A factual qualifier such as `(transitive dependency)`
+   is permitted. Limit optional identification prose to factual component and
+   material scope.
+2. **Upstream copyright statement or statements**, preserving their wording.
+   Put multiple statements in separate paragraphs. Include this field when the
+   selected upstream source supplies an applicable statement; otherwise, omit it.
+3. **SPDX license identifier or expression**, when applicable, using
+   `SPDX-License-Identifier: EXPRESSION`.
+4. **License and notice links**, beginning `License and notices:`, separated
+   by ` | `, and ending with a period. Label local standard texts by license
+   name, such as `MIT` or `Apache 2.0`. Label upstream files by their actual
+   filenames, such as `COPYRIGHT`, `LICENSE`, `NOTICE`, or `PATENTS`.
+5. **Source repository link**, rendered exactly as `[Source](COMMIT_URL).`.
+
+Use the upstream [attribution example](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/examples/licensing/attribution.md)
+to create the project’s `docs/oss/attribution.md`.
+The example illustrates omitted fields and multiple copyrights and licenses.
+Replace the uppercase placeholders and illustrative URLs with actual component
+information. Local license links are relative to the destination
+`docs/oss/attribution.md`; copy the applicable texts into its `licenses/`
+directory. Do not copy example entries for components the project does not use.
+
+These are five fields, not a line limit. Omit the SPDX field when no applicable
+identifier exists; identify nonstandard terms through the linked text. Preserve
+licensing alternatives and exceptions, and record a selected option when
+relevant through the factual license identification. Do not invent an identifier
+or silently resolve an unsettled licensing choice.
+
+Keep entries factual. Do not add agent-authored explanations of license
+expressions, file-level license scope, research performed, retained source
+notices, or compliance conclusions. Avoid filler such as "none is invented
+here" and embellished link labels such as "upstream LICENSE", "additional
+patent grant", or "at the imported commit". Preserve actual required upstream
+content; this rule excludes editorial additions, not required notices. Policy
+interpretation and exception reasoning belong in the adopted OSS policy or an
+appropriate decision record, not the attribution document.
+
+## License Texts And Upstream Links
+
+Link standard license texts to their local copies. Link project-specific
+notices to their upstream copies. Every upstream repository or notice link must
+identify the exact commit corresponding to the
+imported version, including the selected source revision for copied material.
+
+Classify material according to its contents. An upstream `LICENSE`,
+`COPYING`, or similar file may contain project-specific notices or exceptions
+in addition to standard wording. Link both the local standard text and the
+commit-specific upstream file when needed. Make companion license texts directly
+reachable from the entry.
+
+For project-specific license and attribution material, this policy accepts
+direct links to the applicable upstream texts as satisfying reproduction,
+inclusion, or copy requirements. This is the adopted house interpretation.
+Agents must apply it without adding local-copy requirements or seeking repeated
+approval solely because a license uses those terms.
+
+This interpretation concerns notice reproduction. Concrete obligations such as
+marking modified files or supplying corresponding source receive their own
+applicable treatment. Preserve required notices already present in incorporated
+source files; the central attribution entry does not authorize stripping them.
+
+## Discoverability
+
+End the project's README with two friendly statements linking to its actual
+grant in `COPYRIGHT` and the attribution document. ConfigKit's
+[README](../../README.md) contains the two required statements: one linking to
+the project grant and one linking to attribution.
+
+Each new checked-in release document and its hosted download page must end
+with links whose visible text is `COPYRIGHT | LICENSE | ATTRIBUTION`. Use absolute
+URLs to those files at the release's immutable tag or corresponding immutable source
+revision so the links work in both locations. This release navigation is distinct
+from upstream component links, which must use the imported commit.
+
+Reuse the checked-in release document as the hosted release body where supported.
+The release procedure owns concrete URLs and publication mechanics for delivering
+the links through the selected channel. Do not maintain duplicate notice lists.
+These conventions do not by themselves require separate downloadable license
+assets, source bundles, or a different binary package format.
+
+## Maintenance And Exceptions
+
+Update project licensing statements together when the author changes the grant
+or its scope. Update upstream notices in the change that adds, removes, upgrades,
+copies, or changes the distribution of a relevant component. Check the selected source's license
+and notices rather than assuming they are unchanged. Update affected entries,
+commit links, and texts; retain historical versions through Git.
+
+Check affected statements, faithful standard-text copies, and link destinations
+as part of ordinary change review. For a future release tag, check intended file
+paths against the candidate source; the tag need not resolve remotely before
+publication. Do not add publication-time network checks, CI polling, recurring
+legal sign-offs, mandatory scanners, or a full software bill of materials merely
+to implement this policy.
+
+Use the House Format routinely. Raise an exception only for a concrete obligation
+or unresolved rights question it does not address, including any arising from a
+particular distribution channel. Record the issue and selected treatment in the
+adopted OSS policy or an appropriate decision record. Attribution entries
+contain the resulting factual identification and required material, without
+repeating that reasoning. Resolve the issue before the affected use or
+distribution without reopening settled choices.
+Common ownership of projects is neither required nor a blanket exception.
+
+Apply the formatting convention prospectively. Do not rewrite published tags,
+notes, binaries, or checksums to retrofit it. Address actual omissions deliberately
+under the applicable terms and the [release correction policy](release.md#immutability-and-correction).
+Ordinary attribution maintenance and release links are preparation work, not new
+release approval gates.

docs/policy/release.md

@@ -1,42 +1,39 @@
 # Release Policy
 
-**Base Policy Version:** 1.0
+**Base Policy Version:** 1.6
 
-**Last Updated:** 2026-09-23
+**Last Updated**: 2026-10-09
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/e833faddf120c789a9b45348edaf75720de16348/docs/policy/release.md)
 
 ## Purpose
 
-This policy defines the minimum requirements for selecting, validating,
-publishing, verifying, and correcting releases. Its goal is to make every
-release intentional, reproducible from an exact source revision, documented
-for consumers, and safe from retrospective replacement.
+This policy defines a proportionate workflow for validating and publishing an
+intended source revision under a fresh version. Releases should require little
+routine intervention, avoid repeated work, and preserve published contents.
 
-The repository-specific commands that implement these requirements belong in
-`docs/release.md`. Historical release summaries belong in `docs/releases/`.
-Neither document may weaken this policy.
+Adopt stronger verification or reproducibility requirements through an explicit
+project-specific decision justified by the intended audience and support needs.
 
-## ConfigKit applicability
+The repository-specific commands belong in `docs/release.md`. Historical release
+summaries belong in `docs/releases/`. The procedure must agree with the adopted
+policy and link to it rather than introduce additional release requirements.
 
-ConfigKit adopts a source-only Go module release model. The source tag and
-version-matched release note are the declared deliverables. Standalone binaries,
-container images, and separate binary/package-registry publication are not
-required. This does not waive source compilation, module hygiene, testing, or
-local target-build validation. Post-publication consumer retrieval is optional,
-as documented under Policy Exceptions.
+## ConfigKit Applicability
 
-The release procedure must define the module path, authoritative remote,
-supported Go versions and operating-system/architecture targets, prerequisite
-dependency acquisition, and the exact verification commands once established.
-The default tests themselves remain deterministic and offline. Additional
-publication channels require documented deliverables and verification before use.
+ConfigKit publishes a source-only Go module. Its declared deliverables are the
+immutable annotated source tag and version-matched checked-in release note.
+Standalone binaries, container images, hosted release objects, and separate
+registry publication are not required. Additional channels require a deliberate
+update to the policy and procedure.
 
-The Go API, documented YAML/resolution semantics, and structured diagnostic
-contracts all participate in compatibility review. A signature-preserving change
+The Go API, documented YAML and resolution semantics, and structured diagnostic
+contracts participate in compatibility review. A signature-preserving change
 can still break consumers. Human diagnostic wording is stable only where a
-contract expressly promises it. Minimum Go version and platform support changes
-also require an explicit compatibility assessment.
+contract expressly promises it. Minimum Go version and platform support
+changes also require an explicit compatibility assessment.
 
-### Platform support
+### Platform Support
 
 Linux is the release-validated platform. macOS has best-effort support: keeping
 it working remains desirable, but neither native macOS validation nor Darwin
@@ -45,53 +42,67 @@ does not by itself block a release. Voluntary checks and fixes are welcome; ther
 is no guarantee that each release has been validated on macOS.
 
 The [release procedure](../release.md#release-identity-and-support) owns target
-architectures and required Linux checks. This support change applies to upcoming
-releases; historical release notes and their validation records remain unchanged.
-The v0.3.0 release note must explain the change in Compatibility, with version
-selection following this policy's existing compatibility rules.
+architectures and required Linux checks. Historical release notes and their
+validation records remain unchanged.
 
-## Release Model
+## Release Model And Completion
 
-A release consists of:
+A source release identifies one exact committed source revision through an
+immutable version tag and a version-matched, checked-in release note. The
+primary release branch is `main` unless the adopted policy specifies otherwise.
+The procedure must name the authoritative publication remote.
 
-- one exact source commit reachable from the repository's primary branch;
-- one immutable version tag identifying that commit;
-- one version-matched, checked-in release note; and
-- every artifact, image, package, registry entry, or other deliverable the
-  repository declares for that release.
+A successful push of the intended tag, with its source commit available on the
+authoritative primary branch, completes the release. The source commit may be
+pushed before the tag or published together with it through an atomic push.
+The procedure must choose and document the supported approach.
 
-The repository's primary branch is `main` unless an adopted policy documents
-an exception. A release candidate must be committed and pushed to that branch
-before its release tag is published.
+ConfigKit uses sequential publication: first push the committed candidate to
+authoritative `main`, then invoke ReleaseKit once to validate that candidate and
+publish its annotated tag. The [release procedure](../release.md) names
+`origin`; ReleaseKit checks its exact effective push URL before validation and
+tag publication.
 
-The source tag is the authoritative release identity. Published deliverables
-must be traceable to the tagged source and must agree with its version. A
-repository may publish source only. When it publishes additional deliverables,
-its release procedure must enumerate them and define how each is built,
-identified, published, and verified.
+Release commands must finish at successful publication. They must not wait for
+or poll CI, require successful CI status, inspect downloadable artifacts, or
+check consumer installation or Go module availability as completion gates.
+A failed or ambiguous push requires resolving the authoritative remote state
+before declaring success or retrying; successful pushes do not require a
+separate remote read-back gate.
 
-Release automation may validate or publish a release only when its exact
-responsibility is documented. Automation must not silently change the source
-candidate, move a published tag, replace a published deliverable, or repair a
-release in place.
+Remote tag read-back and fresh consumer retrieval are optional and non-gating
+after successful publication. A failed push receives one bounded check of the
+authoritative ref; do not poll or blindly retry.
 
-A successful push of the intended annotated tag completes the source-only release.
-Never wait for CI or use its status as a readiness or completion gate. Optional
-post-publication checks must not delay reporting success.
+Repositories may publish source only. Additional binaries, images, packages,
+checksums, or other declared deliverables may be published asynchronously from
+the tagged source. The procedure must identify those deliverables and the
+responsibility of their automation. Published deliverables must be traceable
+to that source and identify the same version.
+
+ConfigKit currently declares no additional deliverables. Confirmed publication
+of the intended source tag is the completion point. CI, macOS validation,
+hosted delivery, and consumer retrieval are outside the release gates.
 
 ## Versioning And Compatibility
 
 Stable releases use Semantic Versioning in the form `vMAJOR.MINOR.PATCH`, with
-no leading zeroes. Each version must be unused locally and in every configured
-remote or publication channel before publication begins.
+no leading zeroes. Prereleases require an explicit adopted policy defining
+their format and use.
 
-Unless an adopted policy explicitly supports and defines prereleases,
-prerelease versions are not permitted.
+ConfigKit does not support prereleases.
+
+The selected version must be unused locally and on the authoritative remote.
+Do not require queries to every configured remote or publication service.
+A version known to have already been published must not be reused, even if the
+normal guards do not inspect that channel. Adding a distribution channel
+requires a deliberate update to the policy and procedure where needed.
 
 For releases at or after `v1.0.0`:
 
 - a major release may make intentional incompatible changes;
-- a minor release may add backward-compatible behavior; and
+- a minor release may add backward-compatible behavior or raise the Go consumer
+  minimum as described below; and
 - a patch release may make backward-compatible fixes.
 
 Before `v1.0.0`, a minor release may change a documented public, operational,
@@ -99,151 +110,210 @@ configuration, data, integration, or compatibility contract when the release
 note explains the impact and required consumer action. A patch release must
 not intentionally break those contracts within its minor line.
 
-A repository with additional compatibility promises must define them in its
-adopted policy or canonical contract documentation. A release note may explain
-a compatibility change, but it must not redefine the current compatibility
-contract.
+Additional compatibility promises and deliberate exceptions belong in the
+adopted policy or canonical contract documentation. Release notes explain
+changes; they do not redefine current compatibility contracts.
 
-## Release Documentation
+## Consumer Minimum And Release Toolchain
 
-Every release made under this policy must include
-`docs/releases/<version>.md` in the tagged source tree. The filename must use
-the complete version tag, and the document must begin with a title containing
-the product name and that exact version.
+Distinguish the minimum toolchain supported for consumers from the toolchain
+maintainers use to build and validate a release. Record the consumer minimum in
+the language's standard metadata and explain its support meaning in canonical
+documentation. Define the selected release toolchain and its setup commands in
+`docs/release.md`. Toolchain selection and upgrade cadence are repository-specific
+choices.
 
-Every new release note must contain these sections:
+For Go repositories, normally declare the initial stable release of the selected
+series as the consumer minimum, such as `go 1.26.0`. Use a higher patch minimum
+when a dependency or an identified correctness fix requires it, and document
+the reason.
 
-```markdown
-# <Product> vMAJOR.MINOR.PATCH
+For release-policy purposes, successful build and release validation on the
+selected Go patch release are sufficient to presume support across that Go
+series, subject to the declared consumer minimum and concrete evidence of
+incompatibility. Investigate specific compatibility concerns as they arise.
 
-This release ...
+Use the selected release toolchain for candidate validation and release builds,
+including CI jobs performing those tasks. A newer validation patch version alone
+leaves the consumer minimum unchanged.
 
-## Summary
+A minor release may raise the Go consumer minimum, including from one Go series
+to the next, both before and after `v1.0.0`. This is an explicit exception to the
+backward-compatibility requirement for minor releases. Patch releases preserve
+the consumer minimum. Explain an increase's impact and required consumer action
+in the release note. Other incompatible changes remain subject to the versioning
+rules above.
 
-## Compatibility
+### ConfigKit Toolchain Selection
 
-## Upgrade
+For minor and major releases, select the newest stable Go series available in
+the development environment. For patch releases, retain the series recorded in
+`go.mod` and its consumer minimum. Use the latest locally available compatible
+patch in the selected series, with `GOTOOLCHAIN=local`, to build the
+module-selected ReleaseKit CLI and validate the candidate. The same Go
+executable builds the tool and runs its validation commands.
 
-## Changes
-```
+## Release Documentation And Preparation
 
-Use an explicit statement such as "No action required" when a section has no
-substantive content.
+Every new release must include `docs/releases/<version>.md` in the tagged
+source tree. Use the complete version tag as the filename and begin with a
+title containing the product name and that exact version.
 
-Release notes are immutable historical summaries. They should link to current
-canonical documentation for exact behavior. Every affected canonical document
-must be updated in the same candidate; a release note must not substitute for
-current documentation or defer required documentation changes.
+ConfigKit requires the first line `# ConfigKit <version>`. ReleaseKit's
+publication guard checks the release-note H1 for the product and version.
 
-## Candidate Preparation And Validation
+Include a short summary and any relevant compatibility impact or upgrade
+instructions. Choose sections suited to the release and keep the note
+proportionate to the change.
 
-ConfigKit provides one local release checker. Run it once against the final
-committed candidate using one maintained patch of the minimum supported Go line
-on Linux. It checks module hygiene, formatting and whitespace, the full offline
-suite with the race detector, static analysis, and CGO-disabled Linux amd64/arm64
-builds. Examples, documentation links, consumer fixtures, and package boundaries
-are covered by that suite, not separate repeated runs. Native package compilation
-is covered by the test run. A second toolchain is optional and non-gating.
+Keep exact current contracts in their canonical documents and update affected
+documentation with the behavior change. Link to those documents from the note
+where useful. If annotated tags are used, derive their summary from the
+checked-in note.
 
-Commit the release note and associated changes before release validation. Record
-the exact validated commit; revalidate if the candidate changes. Do not require
-both pre-commit and post-commit full checker runs for unchanged content.
-Do not publish uncommitted or unvalidated content. Local check failures still
-block publication. Skipped test cases are reported, not separate human sign-off
-requirements.
+ConfigKit uses the complete checked-in note as the annotated tag message.
 
-Compatibility review, release-note accuracy, removal of completed roadmaps, and
-review for unintended files, credentials, private data, and unrelated changes
-are ordinary preparation. The executing agent may perform them without routine
-human approval or separate manual evidence records. Ask the maintainer only for
-genuine unresolved decisions or missing authority. A clean existing checkout is
-sufficient; a dedicated clone is optional. Dependency and toolchain update
-research belongs to maintenance or relevant changes, not every release.
+Include the release footer defined by the [OSS policy](oss.md#discoverability)
+in each new release document. The release procedure defines the concrete URLs
+and publication mechanics. Prepare these links with the release note; they do
+not add a network check or approval gate. If a hosted download page is used in
+a future channel, preserve the footer there.
+
+Handle version selection, release-note content, and retirement of completed
+roadmaps during preparation. Preserve planned and deferred work and repair
+affected links when retiring roadmaps. Reuse ordinary change review for
+unintended files, credentials, and private data. Scope release-content review
+to the tagged source.
+
+## Candidate Validation
+
+Provide one publication command that validates the final committed candidate,
+checks publication state, and publishes the intended refs. A validation-only
+mode or command may be provided for diagnosis, but must not be a required step
+before publication repeats the same checks.
+
+ConfigKit uses the module-selected ReleaseKit CLI's `publish
+vMAJOR.MINOR.PATCH` command. Its `check` command is optional for diagnosis; it
+is not a required preliminary pass to publication.
+
+Run the complete deterministic, offline default test suite once for the final
+candidate, following the [testing policy](testing.md). For repositories that
+publish builds, verify the required build targets. Select those targets for
+the application's actual support commitments; native testing on each platform
+and access to particular hardware are not default release prerequisites.
+
+Prepare dependencies and build the tool before offline validation; the
+publication command is the required validation run, with no separate successful
+test or CI run required before it.
+
+The selected profile runs the complete deterministic, offline default suite
+once with the race detector and test-time vet disabled. The suite already
+includes examples, consumer fixtures, documentation links, and package
+boundaries. One full vet pass adds static checks beyond the test-time subset.
+CGO-disabled Linux amd64 and arm64 package compilation covers supported
+configurations distinct from native race compilation. macOS and newer-toolchain
+checks are optional.
+
+Additional gates must address a concrete risk that the existing checks do not
+adequately cover. Do not automatically require linting, race testing, module
+hygiene audits, documentation audits, credential scans, example validation, or
+expanded platform matrices solely because they are possible. Retain such
+checks when their distinct protection justifies their cost. A one-time migration
+check must not silently become a permanent release requirement.
+
+ReleaseKit runs its selected Go commands with readonly module mode and
+`GOPROXY=off`; acquire dependencies and build the tool before candidate
+validation. Formatting and whitespace remain ordinary development checks in
+`scripts/check.sh`.
+Review module tidiness when dependencies change, not as a release gate.
+
+Release tests must not require real credentials, mutable application services,
+paid APIs, provider probes, or inference calls. The procedure may describe dependency
+preparation separately from offline validation.
+
+Avoid duplicate validation across release phases. Successful checks for the
+same final candidate need not be repeated solely to satisfy another phase;
+any reuse must clearly apply to that exact candidate. Persistent validation
+caches are not required. If the candidate changes, validate the new candidate.
+Cheap publication-state checks may be repeated immediately before pushing
+because local or remote state may have changed during validation.
+
+Automate mechanical guards. Human review gates are strongly disfavored: add one
+only through an explicit project-specific policy decision explaining the
+concrete risk and why ordinary change review or automation is insufficient.
+Do not add routine confirmations for checks the command can perform itself.
+
+Compatibility review, release-note accuracy, completed-roadmap retirement, and
+review for unintended files or credentials remain ordinary preparation. The
+executing agent may perform them without routine human approval or separate
+manual evidence records. Publication itself requires authorization.
 
 ## Publication Guards
 
-Immediately before publication, the maintainer or release automation must
-fetch the current primary branch and tags from the authoritative remote and
-verify all of the following:
+Automate the checks needed to publish the intended source under a fresh version:
 
-- the version has the required format and is not already published;
-- the current branch is the primary release branch;
-- the worktree and index are clean;
-- the current commit is the recorded, validated candidate;
-- the authoritative remote branch points to that same commit;
-- the matching release note exists in that commit and has the required title
-  and sections;
-- release publication is authorized and the local automated gates succeeded for
-  that commit; and
-- no local or remote tag already uses the selected version.
+- the version has the adopted format and is unused locally and on the
+  authoritative remote;
+- the worktree and index are clean on the primary release branch;
+- the source being published is the exact candidate that passed validation,
+  including its matching release note H1;
+- the remote primary branch can be advanced to that candidate without
+  discarding remote history, and is published before the tag; and
+- the tag identifies the validated candidate and only the intended refs are
+  published.
 
-Create the version tag against the guarded commit and verify its target before
-publishing it. Publish only the exact intended ref and deliverables. Broad
-operations such as `git push --tags` are discouraged because they can publish
-unreviewed refs unrelated to the release.
+ReleaseKit checks that `origin`'s effective push URL exactly matches the
+authoritative URL before candidate validation. This does not assert the
+configured fetch URL or require the release-note H1 to be the first line; the
+required `# ConfigKit <version>` first-line format remains a content rule. The
+command checks the candidate and unused version again after validation, and
+checks local state before creating the tag. Ignored local files do not block
+publication.
 
-The repository-specific procedure must make these guards executable or give
-copyable commands for performing them.
+Fetch the authoritative branch and tag state as needed for these checks.
+Do not force branch or tag updates to bypass a failed guard. Publish explicit
+refs rather than all local tags. Do not require CI success, publication-service
+availability checks, or a separate manual confirmation for each guard.
 
 ## Immutability And Correction
 
-Published version tags and published deliverables are immutable. Do not move,
-overwrite, recreate, or delete a published tag. Do not replace an artifact,
-image, package, checksum, signature, provenance statement, or release note
-under an already published version.
+Published version tags and published contents are immutable. Do not move,
+overwrite, recreate, or delete a published tag, or replace a published artifact,
+checksum, signature, provenance statement, or release note under that version.
+The checked-in note at the immutable tag is that release's historical record.
 
-If validation fails before publication, correct the candidate, rerun the
-complete gates, and repeat the publication guards. An unpublished local tag
-may be removed after confirming that it was never published or used by a
-release channel.
+If validation fails before publication, correct and commit the candidate and
+rerun the publication command. If a push fails or its outcome is ambiguous,
+inspect the authoritative remote before retrying. Remove an unpublished local
+tag only after confirming it was never published or used by a release channel.
+An unavailable remote leaves publication status unknown; do not overwrite a tag
+to resolve uncertainty.
 
-If publication or post-publication verification reveals a defect, leave the
-published release intact. Correct the defect on the primary branch, choose a
-new patch version or other semantically appropriate new version, write a new
-matching release note, and perform the complete release procedure again.
+Transient network, authentication, CI, or consumer-availability failures do not
+by themselves require a new version.
 
-A distribution channel's supported yank, deprecation, or advisory mechanism
-may warn consumers away from a defective release, but it must not replace that
-release's contents or reuse its version.
+A defect requiring changes to published source or contents requires a corrected
+commit, a new semantically appropriate version, a matching release note, and
+the release procedure again. Leave the original release intact. A channel's
+yank, deprecation, or advisory mechanism may warn consumers away from a defective
+release without replacing its contents or reusing its version.
 
 ## Repository Release Procedure
 
-Every adopting repository must maintain `docs/release.md` as the canonical
-maintainer runbook for implementing this policy. It must define, in execution
-order:
+Maintain `docs/release.md` as a short, executable maintainer runbook. It must
+identify prerequisites, the selected release toolchain, the primary branch and
+authoritative remote, version and note preparation, one publication command with
+local checks and guards, source-only deliverables, the successful-tag-push
+completion point, and partial or unknown-state recovery.
 
-1. maintainer prerequisites and required access;
-2. version selection and compatibility review;
-3. release-note creation;
-4. candidate preparation and commit;
-5. one local validation pass, exact candidate recording, and push;
-6. publication guards;
-7. exact tag and deliverable publication commands;
-8. completion on successful tag push and optional post-publication checks; and
-9. failure handling and correction.
-
-The procedure should use copyable, fail-fast commands where practical. It must
-name the primary branch, authoritative remote, tag form, published
-deliverables, supported build targets, release automation responsibilities,
-and optional consumer verification path. It must link to this policy instead of
-restating or redefining its requirements.
+Use copyable commands where practical. Keep optional diagnostics and consumer
+verification clearly optional. The procedure must not turn preparation work,
+ordinary change review, or post-publication investigation into additional
+release gates.
 
 ## Policy Exceptions
 
-Any exception to this policy must be explicitly documented in this section,
-including the repository constraint requiring it, its rationale, scope, and
-compensating controls. Customizing release mechanics must not silently weaken
-the policy's integrity, validation, documentation, verification, or immutability
-requirements.
-
-For ConfigKit's source-only releases, the maintainer accepts successful publication
-of the intended tag as sufficient completion evidence. This is an explicit
-exception to the shared standard's mandatory post-publication verification:
-authoritative tag read-back and fresh consumer retrieval are optional and
-non-gating. CI status and downstream retrieval availability must not delay release
-completion. Compensating controls are local validation of the exact candidate,
-automated pre-publication guards, matching remote main, verification of the local
-annotated tag target, explicit ref pushes, and immutable published versions.
+The following historical exceptions do not change the current release workflow.
 
 For v0.1.1 only, the maintainer approved renaming the reserved `_configkit` key to
 `configkit` as an error-correcting point release because there are no current
@@ -257,6 +327,3 @@ For v0.2.0 only, the maintainer accepted the successful native macOS check at
 only. The final candidate still requires the complete Linux release checker.
 This exception waives only repeating the macOS check after those documentation
 changes; it does not waive other publication or verification gates.
-
-Candidate selection and guarded publication remain mandatory; post-publication
-verification is optional in the [release procedure](../release.md).

docs/policy/testing.md

@@ -1,14 +1,16 @@
 # Testing Policy
 
-**Base Policy Version:** 1.0
+**Base Policy Version:** 1.3
 
-**Last Updated:** 2026-09-06
+**Last Updated**: 2026-09-28
+
+**Upstream Source:** [Standard Repository Policies](https://gitea.maximumdirect.net/eric/policy/src/commit/d84a38ebab26f048cafbe59a2246700c35d29843/docs/policy/testing.md)
 
 ## Purpose
 
 Our tests exist to make **incorrect changes expensive and correct changes cheap**.
 
-We do not optimize for test count, line coverage, exhaustive isolation, or the fewest possible tests. We optimize for sufficient confidence in important behavior while imposing as little unnecessary friction as possible on future development.
+We optimize for sufficient confidence in important behavior while minimizing friction on future development.
 
 ## ConfigKit-specific priorities
 
@@ -44,32 +46,19 @@ policy does not imply that those tools or tests already exist.
 
 ## Every test has a cost
 
-Testing is not an unqualified good. Every test imposes both an immediate cost and a continuing lifetime cost.
-
-A test must be:
-
-- written and reviewed;
-- understood by future maintainers and coding agents;
-- executed in local and CI workflows;
-- diagnosed when it fails;
-- updated when legitimate behavior changes;
-- maintained as fixtures, APIs, and dependencies evolve; and
-- removed or rewritten when it becomes redundant, brittle, misleading, or obsolete.
-
-Tests also create cognitive and architectural friction. They can constrain refactoring, duplicate policy, slow feedback loops, add noise to failures, and cause harmless implementation changes to require unrelated edits across the suite.
+Every test has an immediate and continuing cost: writing, review, execution,
+diagnosis, fixture maintenance, and updates when legitimate behavior changes.
+Tests can also constrain refactoring, duplicate policy, and add failure noise.
 
 A test is warranted only when the confidence it provides justifies these costs.
-
-Apply this cost-benefit analysis at two levels:
+Evaluate that tradeoff at two levels:
 
 1. **Per test:** What realistic defect does this test detect, how consequential would that defect be, and is that protection worth the test's lifetime cost?
 2. **Across the suite:** Does this collection provide materially more confidence than a smaller, simpler suite would?
 
-The preferred test suite is a **lean suite that provides sufficient confidence in the risks that matter, without redundant or low-value tests**. We seek sufficient confidence with the least unnecessary testing friction, not the fewest possible tests.
+Some friction is intentional. Tests should make dangerous changes—such as breaking compatibility, corrupting data, violating security boundaries, or reintroducing subtle bugs—require deliberate review. Keep ordinary internal changes inexpensive.
 
-Some friction is intentional. Tests should make dangerous changes—such as breaking compatibility, corrupting data, violating security boundaries, or reintroducing subtle bugs—require deliberate review. They should not make ordinary internal changes needlessly expensive.
-
-The cost of a test is not a reason to omit testing by default. Do not cite maintenance cost abstractly. When omitting a plausible test, be able to state why the protected failure is low-risk, already covered, obvious, reversible, or cheaper to detect elsewhere. For consequential, subtle, or difficult-to-observe behavior, the presumption should favor testing.
+Evaluate maintenance cost against the specific failure being protected. When omitting a plausible test, be able to state why the protected failure is low-risk, already covered, obvious, reversible, or cheaper to detect elsewhere. For consequential, subtle, or difficult-to-observe behavior, the presumption should favor testing.
 
 ## Default testing style
 
@@ -81,6 +70,8 @@ Use a **classical/Detroit-style** approach:
 - Prefer package-level behavioral tests over tests coupled to private helpers or internal call sequences.
 - Treat exact collaborator interactions as testable behavior only when the interaction itself is a requirement.
 
+Examples of appropriate seams include clocks, randomness, subprocesses, remote APIs, object storage, email, and paid LLM calls.
+
 For ConfigKit, controlled input readers and filesystem failures are useful seams.
 Prefer actual parsing, composition, and decoding over mocks of their call order.
 
@@ -92,7 +83,7 @@ Control clocks, randomness, environment variables, and other process-global or m
 
 ## What deserves tests
 
-Prioritize tests for:
+To the extent the following features are present in the application, prioritize tests for:
 
 1. Public and package-level contracts.
 2. Domain rules and important invariants.
@@ -102,15 +93,27 @@ Prioritize tests for:
 6. Previously observed or plausible regressions.
 7. Representative integration and end-to-end workflows.
 
-A package-level contract is behavior relied upon by another package or major collaborator, not every observable detail of a package implementation.
+A package-level contract comprises behavior relied upon by another package or major collaborator.
 
-For behavior involving **data integrity, destructive operations, compatibility, security, concurrency, idempotency, or recovery**, presume that durable tests are required unless the behavior is already credibly protected at another layer.
+Choose tests for requested behavior and established contracts. Tests must not
+introduce new restrictions, guarantees, or threat scenarios merely because they
+could be asserted. Follow the [agent policy](agents.md) when deciding whether a
+behavior belongs to the contract; current implementation behavior alone is not
+sufficient authority.
+
+For established behavior involving **data integrity, destructive operations,
+compatibility, security, concurrency, idempotency, or recovery**, favor durable
+tests for realistic failures whose consequences warrant protection. Identify
+the failure and the existing contract it would violate. These categories call
+for attention, not an automatic test inventory or permission to strengthen the
+application's guarantees. Account for protection already provided at other layers.
 
 Do not add tests merely because a function, branch, or line exists. Do not add a test when the same meaningful risk is already adequately protected elsewhere.
 
 ## Choose the right test boundary
 
-Test through the narrowest stable boundary that expresses the behavior clearly.
+Choose the narrowest stable boundary that expresses the behavior clearly and
+provides durable confidence with minimal incidental coupling.
 
 This is often the package API, but it may instead be:
 
@@ -118,28 +121,15 @@ This is often the package API, but it may instead be:
 - a package-level operation when several internal collaborators jointly produce the behavior; or
 - a larger integration boundary when correctness emerges from interaction with a real dependency.
 
-Do not force all behavior through oversized end-to-end tests. Do not test every private helper merely because it exists. Choose the boundary that gives durable confidence with the least incidental coupling.
-
 ## Test behavior, not implementation
 
-A test should protect a decision, contract, or invariant—not memorialize the current implementation.
+A test should protect a selected decision, contract, or invariant.
 
-Before adding or retaining a test, ask:
+A test is suspect when it primarily detects changes to private constants or
+helpers, equivalent operation ordering, incidental formatting, equivalent
+algorithms, or internal structure that preserves behavior.
 
-> What realistic defect would this test catch?
-
-A test is suspect when its main purpose is to detect that someone:
-
-- changed an internal constant;
-- renamed or split a private helper;
-- reordered equivalent internal operations;
-- changed incidental formatting;
-- replaced one correct algorithm with another; or
-- refactored internal object structure without changing behavior.
-
-Refactoring should normally require no test edits unless the refactored structure is itself part of the contract.
-
-A test can be factually correct and still have negative value. Accurately describing current behavior is not enough; the protected behavior must be important enough to justify the future friction.
+Behavioral tests should normally remain stable through internal refactoring that preserves the contract.
 
 ## Expected effects of different changes
 
@@ -156,12 +146,13 @@ A test failing is not the same as a test needing to be edited. Many tests may co
 
 ## Separate mechanism from policy
 
-Configurable thresholds and defaults must not be duplicated throughout the test suite.
+Derive expectations from test-controlled configuration and relationships.
 
-For example, do not encode an internal concurrency limit indirectly:
+For this example, assume the selected contract rejects a request to start more
+processes than a configured limit. Do not encode its production default indirectly:
 
 ```go
-// Production policy:
+// Production default:
 const maxConcurrency = 4
 
 // Brittle test:
@@ -179,25 +170,30 @@ require.NoError(t, runner.Start(limit))
 require.ErrorIs(t, runner.Start(limit+1), ErrTooMuchConcurrency)
 ```
 
-The test should prove:
+For this selected contract, the test should prove:
 
 - the configured limit is accepted; and
 - one beyond the configured limit is rejected.
 
+Rejection is not a general requirement for concurrency control. If the selected
+contract queues excess work or limits active workers while accepting larger
+batches, test that behavior instead. Do not add rejection or a configurable
+limit solely to reproduce this example.
+
 The production default should be tested exactly only when its literal value is itself a public, operational, safety, protocol, or compatibility requirement.
 
-Apply the same rule to limits, timeouts, capacities, retry counts, and ranges: test relationships and behavior, not duplicated literals.
+Apply the same rule to limits, timeouts, capacities, retry counts, and ranges: test relationships and behavior.
 
-For concurrency limits, test both kinds of behavior when relevant:
+For established concurrency limits, distinguish these behaviors where the contract requires them:
 
-1. **Configuration enforcement:** invalid or excessive requested values are handled correctly.
+1. **Configuration handling:** values are accepted or rejected according to the selected contract.
 2. **Runtime enforcement:** observed peak concurrency never exceeds the configured limit.
 
-Use a test-controlled limit and measure the behavior relative to that limit. Do not merely assert today's default value.
+Use a test-controlled limit and measure the behavior relative to that limit.
 
-## Avoid semantic duplication across layers
+## Assign each behavior a test owner
 
-Each behavior should have a clear test owner.
+For ConfigKit, assign ownership as follows:
 
 - YAML boundary tests own syntax and policy enforcement.
 - Composition and overlay tests own their operation semantics.
@@ -209,9 +205,7 @@ Each behavior should have a clear test owner.
 
 These are responsibility boundaries, not a required package layout.
 
-Higher-level tests should not repeat every lower-level case. A single intentional policy change should not require unrelated edits across many test files.
-
-Tests that are individually reasonable may still be collectively redundant. Evaluate the marginal value of each additional test in light of the protection already provided by the rest of the suite.
+Give each layer its distinct responsibility and keep expectation updates localized to the contract being changed.
 
 ## Use test doubles deliberately
 
@@ -232,104 +226,77 @@ Mocks are appropriate when the contract includes facts such as:
 - an expensive API is called no more than once; or
 - a security audit event is emitted.
 
-Do not use mocks merely to isolate every object or reproduce the implementation's call graph.
-
 ## Go-specific guidance
 
-Use:
+Select techniques by behavior and risk:
 
 - table-driven tests for meaningful behavioral categories and boundaries;
 - `t.TempDir()` for real filesystem behavior;
-- fuzz tests for parsers, normalization, path handling, and broad input spaces;
+- `httptest.Server` for realistic HTTP interactions;
+- fuzz tests when exploring broad input spaces, such as parsers or path handling, adds useful protection beyond representative cases;
 - golden files only when the complete output is intentionally stable;
 - integration tests where correctness depends on component interaction; and
-- a small number of representative end-to-end tests.
+- a small number of representative end-to-end tests when assembled workflows expose meaningful risks not adequately covered at narrower boundaries.
 
-Avoid exact error-string assertions unless the wording is itself contractual. Prefer `errors.Is`, `errors.As`, typed errors, or structured error fields.
+Prefer `errors.Is`, `errors.As`, typed errors, or structured error fields. Use exact error-wording assertions only when the wording is contractual.
 
 At the public library boundary, prefer diagnostic codes, paths, and source
 locations over complete message snapshots. Assert wording only when contractual.
 
 Golden-file updates must require an explicit local flag. CI must not update golden files automatically, and reviewers must inspect the semantic diff before accepting an update.
 
-Keep tests readable and direct. Test helpers and fixture frameworks must earn their own maintenance cost; do not build elaborate test infrastructure for small or isolated needs.
+Keep tests readable and direct. Size test helpers and fixture frameworks to the maintenance benefit they provide.
 
 ## Coverage
 
-Coverage is a diagnostic, not a target.
+Use coverage to locate untested critical branches and unexpectedly weak packages. Evaluate test quality through meaningful behavioral protection and choose coverage according to risk.
 
-Use it to find untested critical branches and unexpectedly weak packages. Do not write low-value tests solely to increase a percentage, and do not infer test quality from coverage alone.
-
-Dense configuration semantics will often warrant higher coverage than simple input wiring. Uneven coverage is acceptable when it reflects risk.
-
-Increasing coverage is valuable only when the newly covered behavior protects a meaningful risk at an acceptable cost.
+Pure domain logic will often warrant higher coverage than CLI wiring or external adapters. Uneven coverage is acceptable when it reflects risk.
 
 ## Regression tests
 
 A bug fix should normally include a regression test that fails before the fix and passes afterward.
 
-Retain the test when the defect could realistically recur and its consequences justify the ongoing cost. Prefer the narrowest durable test of the violated contract or invariant; do not preserve accidental implementation details from the original bug.
+Retain the test when the defect could realistically recur and its consequences justify the ongoing cost. Prefer the narrowest durable test of the violated contract or invariant.
 
-Not every historical bug requires a permanent test. If the underlying design has made recurrence impossible, the test has become redundant, or a stronger invariant test now subsumes it, remove or consolidate it.
+Remove or consolidate a test when the underlying design has made recurrence impossible, its protection is redundant, or a stronger invariant test subsumes it.
 
 ## Deleting or rewriting tests
 
-Tests are maintained code, not permanent historical artifacts.
-
-Delete or rewrite a test when its maintenance cost exceeds the confidence it provides.
+Delete or rewrite tests when maintenance cost exceeds their continuing
+protective value.
 
 Strong candidates include tests that:
 
 - require updates after harmless internal changes;
-- directly assert private constants without protecting a real contract;
-- duplicate the same policy across several layers;
+- assert incidental private constants or snapshot large amounts of incidental output;
 - verify mock choreography rather than outcomes;
-- snapshot large amounts of incidental output;
-- test trivial private helpers already exercised through stable package behavior;
-- protect risks already covered more effectively elsewhere;
+- duplicate policy across layers, retest trivial helpers covered by package
+  behavior, or protect risks covered more effectively elsewhere;
 - are flaky, misleading, obsolete, or disproportionately expensive to diagnose; or
 - no longer correspond to a plausible failure mode.
 
-Several brittle tests may encode one genuine requirement. Replace them with one durable behavior-level or invariant test rather than preserving all of them.
-
-Deleting a low-value test can improve the quality of the suite by reducing noise, maintenance burden, and friction around legitimate change.
+Several brittle tests may encode one genuine requirement. Consolidate them into one durable behavior-level or invariant test.
 
 ## Reviewing a proposed test
 
-Use the following questions when the value, boundary, or durability of a proposed test is not self-evident. Significant test additions should be reviewable against them, but written answers are not required for every routine test.
+Use the following questions when a proposed test needs closer evaluation. Significant test additions should be reviewable against them; written answers are optional for routine tests.
 
-1. What realistic defect would it catch?
-2. How likely is that defect?
-3. How consequential would it be?
-4. Is the behavior already protected elsewhere?
-5. At which layer should this behavior be owned?
-6. Does the test assert a durable contract or an incidental implementation detail?
-7. Could the implementation be refactored without changing the behavior and without editing this test?
-8. What should cause this test to fail?
-9. What legitimate changes should not cause this test to fail?
-10. What ongoing maintenance, execution, and diagnostic cost will the test impose?
-11. Is there a smaller or more direct test that protects the same risk?
-
-Do not add the test when its expected lifetime cost exceeds its expected protective value.
-
-When deciding not to test plausible behavior, record or be able to explain why the risk is low, already protected, obvious, reversible, or cheaper to detect elsewhere.
+1. Which requested behavior or established contract does it protect, and what realistic defect would violate it?
+2. How likely and consequential is that defect, and is it already protected elsewhere?
+3. What is the narrowest stable boundary that can test it clearly?
+4. Would a correct internal refactor or a change to an incidental default require test edits?
+5. Does its confidence justify its maintenance, execution, and diagnostic cost, or would a smaller test suffice?
 
 ## Definition of sufficient
 
 A test suite is sufficient when:
 
-- important contracts and invariants are protected;
-- meaningful boundaries and failure modes are exercised;
-- realistic and consequential regressions are credibly protected against silent recurrence;
-- behavior involving data integrity, destructive operations, compatibility, security, concurrency, idempotency, and recovery is credibly protected;
-- important external boundaries have realistic integration coverage;
-- representative complete workflows are tested;
-- failures provide useful signal rather than redundant noise;
-- legitimate internal changes usually do not require test edits; and
+- important requested behavior and established contracts are protected against realistic, consequential failures;
+- boundaries and component interactions have coverage where they introduce meaningful risks not protected elsewhere;
+- failures provide useful, distinct signals;
+- tests usually remain stable through legitimate internal changes; and
 - additional tests would mostly repeat existing protection or preserve inconsequential implementation details.
 
-Sufficiency is a risk judgment, not a coverage percentage or test count. Reassess it as the library, its consumers, and the consequences of failure evolve.
-
-The governing rule is:
-
-> Test heavily where failure is consequential, subtle, or difficult to detect after the fact. Test lightly where failure is obvious, reversible, and inexpensive—and retain no test whose lifetime cost exceeds the confidence it provides.
+Reassess sufficiency as the application, its users, and the consequences of
+failure evolve.

docs/release.md

@@ -1,237 +1,237 @@
 # ConfigKit Release Procedure
 
 This is the maintainer runbook for ConfigKit's [release policy](policy/release.md).
-ConfigKit publishes one source-only Go module: the immutable source tag and the
-matching checked-in release note are the deliverables. There is no application
-binary, container image, generated package, or separate registry publication.
+ConfigKit publishes a source-only Go module. The immutable annotated tag and
+matching checked-in release note are the deliverables; there is no binary,
+container image, generated package, or separate registry publication.
 
 ## Release identity and support
 
 - Module: `gitea.maximumdirect.net/eric/ConfigKit`
 - Primary branch: `main`
-- Authoritative remote: `origin`, expected to fetch from
-  `https://gitea.maximumdirect.net/eric/ConfigKit.git`
-- Tags: stable Semantic Versions such as `v0.1.0`; prerelease tags are not supported
-- Minimum Go language/toolchain line: Go 1.25, release-validated with one maintained
-  patch of that line; newer-toolchain testing is optional and non-gating
-- Release-gated build targets: Linux on amd64 and arm64
-- Best-effort targets: macOS on amd64 and arm64, under the
+- Authoritative remote: `origin`
+- Authoritative push URL: `https://gitea.maximumdirect.net/eric/ConfigKit.git`
+- Tags: stable Semantic Versions such as `v0.5.0`; no prereleases
+- Minimum Go version: the first stable patch of the selected Go series, or a
+  higher patch when required by a dependency or specific correctness fix, as
+  recorded in `go.mod`; see the
+  [consumer minimum policy](policy/release.md#consumer-minimum-and-release-toolchain)
+- Release-gated targets: Linux amd64 and arm64
+- Best-effort targets: macOS amd64 and arm64, under the
   [platform support policy](policy/release.md#platform-support)
 
-The local Linux checker includes native race tests and CGO-disabled cross-builds.
-Its output reports skipped filesystem cases; no separate evidence record or
-human sign-off is required. Native macOS checks and Darwin cross-builds are
-optional and do not gate publication.
+Release validation runs one offline race-enabled Go suite, one full vet pass,
+and CGO-disabled package compilation for both Linux targets. macOS and CI are
+not release gates; do not wait for or poll CI.
 
-CI is never a release gate. Do not wait for it, poll its status, or delay reporting
-completion because it is pending, unavailable, or failed.
+## 1. Select Go and prepare the tool
 
-## 1. Prepare access and tools
+Use a clean checkout with Git, Bash 3.2 or newer, and a native compiler for race
+tests. Choose the release type before selecting Go. For a minor or major release,
+select the newest stable Go series available in the development environment.
+For a patch release, retain the series recorded in `go.mod` and leave its consumer
+minimum unchanged. Follow the
+[toolchain selection policy](policy/release.md#configkit-toolchain-selection).
 
-The release maintainer owns this procedure. Use a clean checkout (a dedicated clone is optional) with Git, Bash 3.2 or newer,
-a maintained Go 1.25 patch, and the compiler needed for native race tests. The checker supports the stock macOS
-Bash; its filename enumeration is null-delimited, including names with whitespace. Configure private-host Git and Go module
-access through the normal credential manager before starting. Keep tokens, passwords,
-SSH material, and environment dumps outside the repository and command transcript.
-Verify access without displaying credentials:
+Put the selected series' latest locally available compatible patch first on
+`PATH`, ensuring it meets `go.mod`'s minimum. The same `go` executable must build
+ReleaseKit and run its child validation commands. Set local toolchain selection
+and verify it:
 
 ```sh
-git remote get-url origin
-git ls-remote origin HEAD
+export GOTOOLCHAIN=local
+go version
+go env GOVERSION
+command -v go
 ```
 
-The remote URL must equal the authoritative URL above. Acquire the selected
-toolchain and dependencies explicitly. For example, with the maintained patch
-used when this procedure was simplified:
+Only when a minor or major release advances the consumer minimum to a newer Go
+series, derive that series' initial stable release from the selected executable:
 
 ```sh
-export CONFIGKIT_BASELINE_TOOLCHAIN=go1.25.14
-GOTOOLCHAIN="$CONFIGKIT_BASELINE_TOOLCHAIN" go version
-GOTOOLCHAIN="$CONFIGKIT_BASELINE_TOOLCHAIN" go mod download
+selected_go=$(GOTOOLCHAIN=local go env GOVERSION)
+selected_series=${selected_go#go}
+selected_series=${selected_series%.*}
+release_minimum="$selected_series.0"
+go mod edit -go="$release_minimum"
 ```
 
-Keep toolchains maintained, but do not require latest-version discovery for each
-release. The checker sets `GOPROXY=off` and fails if the cache is incomplete.
-Toolchain and YAML-dependency update research belongs to periodic maintenance or
-relevant dependency changes. No renewed approval of an unchanged pin is required.
+If a dependency or identified correctness fix requires a higher patch minimum,
+use that required version instead and document the reason. When the series is
+unchanged, leave the minimum unchanged unless a minor or major release
+deliberately changes it under the
+[consumer minimum policy](policy/release.md#consumer-minimum-and-release-toolchain).
+For example, validating with Go 1.26.8 leaves `go 1.26.0` unchanged. There is no
+separate requirement to validate with Go 1.26.0.
 
-Use Bash with `set -euo pipefail` for the command blocks below so any failed gate
-stops the sequence. Publication requires user authorization; the agent policy's
-local-commit authorization is not permission to push or release.
-
-## 2. Select the version and review compatibility
-
-Choose an unused stable version and compare the candidate with the previous tag.
-For the first release, review the whole public surface and current contracts.
+Prepare dependencies whether or not the minimum changes:
 
 ```sh
-release_version=v0.1.0
-printf '%s\n' "$release_version" | grep -Eq '^v(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)$'
-if git show-ref --verify --quiet "refs/tags/$release_version"; then exit 1; fi
-if git ls-remote --exit-code --tags origin "refs/tags/$release_version" >/dev/null; then
-  printf 'remote tag already exists: %s\n' "$release_version" >&2
-  exit 1
-else
-  test $? -eq 2
-fi
-git log --oneline --decorate --no-merges
+go mod download
 ```
 
-Review exported Go signatures and ownership, YAML and scalar behavior, precedence,
-diagnostic codes, minimum Go version, and platform support. Before `v1.0.0`, any
-intentional incompatible minor-release change must be explained with consumer action;
-patch releases must remain compatible within their minor line. Review the maintained
-consumer examples and the migration guide even when signatures did not change.
-
-When dependencies change, review parser behavior and migration impact as part of
-that change. Keep Compatibility accurate about the dependency pin, but do not
-perform an external version survey for every patch release.
-
-## 3. Write the release note
-
-Create `docs/releases/$release_version.md` before selecting the candidate. It must
-begin with `# ConfigKit $release_version` and contain `Summary`, `Compatibility`,
-`Upgrade`, and `Changes` sections. Use “No action required” where appropriate. Link
-to current canonical contracts instead of copying their definitions. Do not create a
-release note until there is a real candidate and reviewed content.
-
-## 4. Validate the source candidate
-
-Before release validation, finish compatibility review, inspect the diff and file
-list for unintended content or credentials, retire completed roadmaps, repair
-links, and review the release note. These are ordinary preparation tasks the
-executing agent may perform, not separate human approvals or evidence records.
-Ask the maintainer only for an unresolved product decision or missing authority.
-
-Commit the candidate, staging only its intended changes, then run the checker
-once against that clean commit. Do not run both everyday and release checkers to
-repeat the same gates.
+Build the tool selected by `go.mod` into temporary storage outside the checkout.
+Ensure `$TMPDIR` names a system temporary location, not the repository. These
+commands never install another ReleaseKit version:
 
 ```sh
-git status --short
-# Stage the specific files belonging to the candidate, then review:
-git diff --staged
-git commit --message "Prepare ConfigKit $release_version"
-test -z "$(git status --porcelain=v1 --untracked-files=all)"
-candidate_commit=$(git rev-parse HEAD)
-./scripts/release-check.sh
-test "$(git rev-parse HEAD)" = "$candidate_commit"
-test -z "$(git status --porcelain=v1 --untracked-files=all)"
-printf '%s\n' "$candidate_commit" > "$(git rev-parse --git-path configkit-release-candidate)"
+releasekit_tmp=$(mktemp -d "${TMPDIR:-/tmp}/configkit-releasekit.XXXXXX")
+releasekit_bin="$releasekit_tmp/releasekit"
+go build -o "$releasekit_bin" gitea.maximumdirect.net/eric/ReleaseKit/cmd/releasekit
+go list -m gitea.maximumdirect.net/eric/ReleaseKit
+"$releasekit_bin" version
 ```
 
-If the final candidate is already committed, omit the staging and commit commands;
-do not create an empty commit. The checker validates module manifests in a
-disposable copy, formatting, whitespace, the full offline suite once with the race
-detector and verbose skip reporting, vet once, and CGO-disabled Linux amd64/arm64
-builds. The suite includes examples, consumer fixtures, documentation links, and
-package boundaries; no separate reruns are required. No additional linter is
-claimed. Sibling-consumer links are checked when those checkouts exist and are
-otherwise inapplicable in an isolated checkout.
+The module-selected executable is ReleaseKit v0.3.0. Use the built executable
+for publication to preserve its exit status and publication-state diagnostics.
+`go run` maps a nonzero child exit status to 1. ReleaseKit's child Go commands
+use the `go` found on `PATH` with `GOTOOLCHAIN=local`; keep the selected
+executable first on `PATH` through the publication command.
 
-A changed candidate requires validation again. Do not reuse a recorded candidate
-from an older revision. The record is written only after successful local checks.
-
-## 5. Prepare publication
-
-The following guarded sequence pushes only `main` and the selected tag. Local
-validation failures block it; CI does not. Keep the selected `release_version`
-from step 2 in the same shell. If starting a new shell, set it explicitly again.
-
-## 6. Run publication guards
-
-Immediately before tagging, fetch the authoritative branch and tags, restore the
-recorded variables if this is a new shell, and execute every guard:
+Verify access to the authoritative remote without displaying credentials:
 
 ```sh
-candidate_commit=$(cat "$(git rev-parse --git-path configkit-release-candidate)")
-git fetch --prune origin main
-git fetch --tags origin
-test "$(git branch --show-current)" = main
-test -z "$(git status --porcelain=v1 --untracked-files=all)"
-test "$(git rev-parse HEAD)" = "$candidate_commit"
-test "$(git remote get-url origin)" = 'https://gitea.maximumdirect.net/eric/ConfigKit.git'
-printf '%s\n' "$release_version" | grep -Eq '^v(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)$'
-test -f "docs/releases/$release_version.md"
-test "$(sed -n '1p' "docs/releases/$release_version.md")" = "# ConfigKit $release_version"
-for heading in Summary Compatibility Upgrade Changes; do
-  grep -Fxq "## $heading" "docs/releases/$release_version.md"
-done
-if git show-ref --verify --quiet "refs/tags/$release_version"; then exit 1; fi
-if git ls-remote --exit-code --tags origin "refs/tags/$release_version" >/dev/null; then
-  printf 'remote tag already exists: %s\n' "$release_version" >&2
-  exit 1
-else
-  test $? -eq 2
-fi
-git push origin HEAD:refs/heads/main
-git fetch origin main
-test "$(git rev-parse refs/remotes/origin/main)" = "$candidate_commit"
-test "$(git rev-parse HEAD)" = "$candidate_commit"
-test -z "$(git status --porcelain=v1 --untracked-files=all)"
+git ls-remote https://gitea.maximumdirect.net/eric/ConfigKit.git HEAD refs/heads/main
 ```
 
-The recorded local validation must apply to exactly `$candidate_commit`. No
-separate human sign-off, CI status, or manual evidence document is required.
+Publication requires explicit authorization. Local implementation commits do
+not authorize a push or release.
 
-## 7. Tag and publish the exact ref
+## 2. Prepare and commit the candidate
 
-Create one annotated tag at the guarded commit, verify its peeled target, and push
-only that ref. Do not use `git push --tags`.
+Choose an unused stable version locally and on the authoritative remote. Compare
+the candidate with the previous tag and review exported Go signatures and
+ownership, YAML and resolution semantics, diagnostic contracts, minimum Go
+version, and platform support. Review maintained consumer examples and the
+migration guidance. Explain any consumer-minimum increase and required consumer
+action under the
+[consumer minimum policy](policy/release.md#consumer-minimum-and-release-toolchain).
+
+Create `docs/releases/<version>.md`. Its first line must be exactly
+`# ConfigKit <version>`, and it must end with the license and attribution footer:
+
+```markdown
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/VERSION/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/VERSION/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/VERSION/docs/oss/attribution.md)
+```
+
+Replace `VERSION` with the full tag and confirm the three target files exist in
+the candidate. Retire completed roadmaps, preserve unfinished and deferred
+work, and repair affected links. Review the complete candidate for unintended
+files and credentials, then commit it on `main`. These are ordinary preparation
+tasks, not separate approval gates. Any `go.mod` change belongs in this
+candidate.
+
+## 3. Push the candidate to `main`
+
+Push only that committed candidate to the literal authoritative URL. This does
+not follow local tags:
 
 ```sh
-git tag --annotate "$release_version" --message "ConfigKit $release_version" "$candidate_commit"
-test "$(git rev-list -n 1 "$release_version")" = "$candidate_commit"
-git push origin "refs/tags/$release_version:refs/tags/$release_version"
+git push --no-follow-tags https://gitea.maximumdirect.net/eric/ConfigKit.git HEAD:refs/heads/main
 ```
 
-A successful push of the intended tag completes the release. Report success now;
-do not wait for CI, module propagation, or any post-publication check. No Gitea
-release object or binary upload is required.
+ReleaseKit requires authoritative `main` to equal this candidate before it
+validates. A failure before this push means ReleaseKit publication has not
+started. If the push result is uncertain, inspect authoritative `main` and
+continue only when it is confirmed at the candidate. If validation later fails,
+no release tag is published; correct the issue in a new commit, push the new
+candidate to `main`, and invoke validation again.
 
-## 8. Optional post-publication checks
+## 4. Publish the committed candidate
 
-Only when useful or requested, read back the remote tag or retrieve the exact
+With the authorized candidate already on authoritative `main`, invoke the
+module-selected native executable exactly once:
+
+```sh
+"$releasekit_bin" publish vMAJOR.MINOR.PATCH
+```
+
+Replace the placeholder with the selected version. ReleaseKit checks the
+effective push URL against `expected_push_url`, verifies the candidate on
+`main`, and checks the release-note H1 for the product and version. It then
+runs the selected race-enabled standard suite once with test-time vet disabled,
+one full vet pass, and CGO-disabled package compilation for Linux amd64 and
+arm64. It publishes only the annotated tag for that candidate. Confirmed
+publication of the intended tag completes the release. The tag points to the
+captured commit and uses the complete checked-in note as its annotation message.
+The optional `check`
+command is available for diagnosis but is not a separate required pass:
+
+```sh
+"$releasekit_bin" check vMAJOR.MINOR.PATCH
+```
+
+Status 0 confirms publication, status 1 reports a known publication failure or
+conflict, status 2 reports usage, profile, or version errors, and status 3 means
+the push outcome remains unknown. Preserve both the native exit status and its
+diagnostics.
+
+The command does not push or repair `main`, wait for CI, or publish binaries or
+a hosted release page. A successful intended-tag publication does not require
+remote read-back or consumer retrieval; those checks remain optional.
+
+## 5. Recover from a failed or uncertain tag push
+
+The executable reports the publication state, phase, version, candidate commit,
+tag-object ID, effective endpoint, and error on standard error. It performs one
+bounded authoritative reconciliation after a failed tag push. Exit status 3
+means the tag outcome remains unknown; do not blindly retry or move a tag.
+
+For an uncertain result, use the exact endpoint, candidate commit, version, and
+tag-object ID from the report. Copy the captured endpoint into the variable
+below using normal shell quoting. Check that Git still resolves it to itself,
+then query the authoritative refs once:
+
+```sh
+authoritative='COPY_CAPTURED_ENDPOINT'
+release_version='vMAJOR.MINOR.PATCH'
+test "$(git ls-remote --get-url -- "$authoritative")" = "$authoritative" &&
+  git ls-remote --refs -- "$authoritative" refs/heads/main "refs/tags/$release_version"
+```
+
+A failed URL-equality check or remote query leaves the outcome unresolved.
+Correct transport configuration before continuing; a rewritten destination
+cannot establish absence at the captured endpoint.
+
+Compare the `main` object ID with the reported candidate commit and the tag
+object ID with the reported tag-object ID. `--refs` returns the annotated tag
+object itself, not merely its peeled commit. If the tag object matches, the
+release is published. If the query fails, the result is conflicting, or the
+endpoint or candidate differs, stop and resolve the state before retrying. If
+the endpoint is reachable, the tag is confirmed absent, and `main` still equals
+the reported candidate, remove a retained local tag only after confirming it
+was not published or used by another channel; then rerun `publish`, which
+revalidates the candidate:
+
+```sh
+git tag -d "$release_version"
+"$releasekit_bin" publish "$release_version"
+```
+
+A validation failure after the candidate reached `main` leaves an untagged
+candidate there. Correct it with a new commit rather than rewriting or rolling
+back `main`. A conflicting published tag must never be moved or reused. A
+defect in published source requires a corrected commit, a new semantically
+appropriate version, and a matching release note.
+
+## 6. Optional consumer verification
+
+When useful or requested, read back the remote tag or retrieve the exact
 version into a new module without a local `replace` directive. These checks are
-optional and non-gating; their failure or unavailability does not delay release
-completion. Do not poll CI or wait for module propagation. Private-host credentials
-must already be configured. Example direct-origin verification:
+optional and non-gating. Do not poll CI or wait for module propagation. For
+direct-origin verification, use the exact published version in a temporary
+consumer module:
 
 ```sh
-remote_tag_commit=$(git ls-remote origin "refs/tags/$release_version^{}" | awk '{print $1}')
-test "$remote_tag_commit" = "$candidate_commit"
+release_version=vMAJOR.MINOR.PATCH
 consumer_dir=$(mktemp -d)
 (
   cd "$consumer_dir"
   go mod init example.invalid/configkit-release-check
   GOPRIVATE=gitea.maximumdirect.net GOPROXY=direct go get "gitea.maximumdirect.net/eric/ConfigKit@$release_version"
-  cat > main.go <<'EOF'
-package main
-
-import configkit "gitea.maximumdirect.net/eric/ConfigKit"
-
-func main() {
-        _ = configkit.NoSource()
-}
-EOF
-  gofmt -w main.go
-  go test ./...
-  go build ./...
   go list -m -f '{{.Path}} {{.Version}}' gitea.maximumdirect.net/eric/ConfigKit
 )
-rm -rf -- "$consumer_dir"
 ```
 
-If performed, the final `go list` output should show the exact module path and
-`$release_version`. Report optional verification separately from publication success.
-
-## 9. Handle failures without replacement
-
-Before publication, fix and commit the candidate, validate it, record the new
-exact candidate, and repeat the guarded publication sequence. An unpublished local tag may be deleted only after verifying it
-was never pushed or used by another release channel.
-
-After publication, never move, delete, or recreate the tag and never replace its
-release note. Correct the defect on `main`, choose a new semantically appropriate
-version, and repeat this procedure. A supported advisory or deprecation mechanism
-may warn consumers about the old version but cannot change its contents.
+The output should show the exact module path and selected version. Report this
+optional check separately from publication success.

docs/releases/v0.4.1.md

@@ -0,0 +1,42 @@
+# ConfigKit v0.4.1
+
+This release formalizes ConfigKit's licensing and upstream attribution
+documentation without changing library behavior.
+
+## Summary
+
+ConfigKit now includes an explicit LGPL-3.0-or-later grant in COPYRIGHT,
+complete LGPLv3 and incorporated GPLv3 license texts, and centralized upstream
+attribution. README and release guidance provide navigation to those materials.
+
+## Compatibility
+
+No Go code, public API, configuration semantics, diagnostic behavior, or
+dependencies changed from v0.4.0. The minimum Go version remains 1.25, Linux
+remains release-validated, and macOS remains best-effort. The YAML dependency
+remains `go.yaml.in/yaml/v4 v4.0.0-rc.6`.
+
+The project grant is explicitly LGPL-3.0-or-later. Third-party material remains
+subject to its respective terms, recorded in the attribution document below.
+Published earlier releases are unchanged.
+
+## Upgrade
+
+Update the module dependency and rebuild to select this release:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.4.1
+```
+
+No application-code or configuration migration is required from v0.4.0.
+
+## Changes
+
+- Add the project COPYRIGHT statement and complete standard project license texts.
+- Document go-yaml attribution with exact upstream revision links and local
+  Apache-2.0 and MIT license texts.
+- Adopt the house OSS policy and connect licensing ownership and contributor guidance.
+- Add license and attribution navigation to README and future release documents,
+  without adding release gates or changing publication mechanics.
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.1/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.1/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.1/docs/oss/attribution.md)

docs/releases/v0.4.2.md

@@ -0,0 +1,45 @@
+# ConfigKit v0.4.2
+
+This patch release simplifies and safeguards source publication and aligns
+maintainer policy and licensing guidance. Library behavior is unchanged.
+
+## Summary
+
+Maintainers can now publish a prepared release with one guarded command. It
+validates the exact committed candidate, checks the authoritative destination,
+and pushes only the candidate on `main` and its annotated version tag. The
+[release procedure](../release.md) explains preparation and partial-publication
+recovery; successful tag publication completes the source-only release without
+waiting for CI or consumer retrieval.
+
+## Compatibility
+
+No Go code, public API, configuration semantics, diagnostic behavior, or
+dependencies changed from v0.4.1. The minimum Go version remains 1.25. Linux
+amd64 and arm64 remain release-validated; macOS remains best-effort. The YAML
+dependency remains `go.yaml.in/yaml/v4 v4.0.0-rc.6`.
+
+## Upgrade
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.4.2
+```
+
+No application-code or configuration migration is required from v0.4.1.
+
+## Changes
+
+- Add guarded single-command publication with exact candidate, version, and
+  remote checks; note-derived annotated tags; and bounded failed-push recovery.
+- Keep one offline race suite, one full vet pass, and Linux amd64/arm64 builds
+  as release gates. Formatting and whitespace remain development checks;
+  dependency tidiness is reviewed when dependencies change.
+- Align adopted policies and contributor guidance with their pinned upstream
+  revision, and include the CC BY-SA 4.0 license text and attribution for
+  adapted policy material. The project grant remains LGPL-3.0-or-later.
+- Retire completed release-workflow and policy-alignment planning records while
+  preserving their history in Git. Deferred scope remains documented.
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.2/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.2/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.4.2/docs/oss/attribution.md)

docs/releases/v0.5.0.md

@@ -0,0 +1,53 @@
+# ConfigKit v0.5.0
+
+This minor release adds source-backed layered configuration and built-in sibling
+inheritance, so applications can load one complete configuration while retaining
+presence, source context, and explainable derivation.
+
+## Summary
+
+- `LoadLayered` combines ordered underlay groups beneath the main input, selected
+  overlay, and explicit CLI overrides. Inputs within a group still compose
+  strictly; precedence tiers combine mappings recursively. The main input
+  remains the owner of the optional schema-version check.
+- `NewFilesystem(...).File(...)` accepts caller-owned `fs.FS` sources, and
+  `Mapping(...)` snapshots supported built-in values as a labeled source.
+  `Result.SourceContext` and `Resolution.SourceContext` expose passive resource
+  contexts for values from file-backed sources.
+- `extends` now resolves a mapping against one named sibling after source
+  precedence and before typed decoding. Inherited values count as explicit
+  input. Origins, history, explanations, and redacted rendering retain the
+  source and the structural path by which a value reached its destination.
+  Review corrections preserve immutable nested-history evidence and bounded
+  expansion work.
+
+See the [configuration contract](../config.md#layered-loading),
+[inheritance rules](../config.md#sibling-inheritance), and
+[consumer examples](../consumers/README.md) for current behavior.
+
+## Compatibility and upgrade
+
+This is an intentional pre-v1 incompatible minor release for configurations
+that use `extends` as ordinary data. That key is now reserved wherever an input
+mapping occurs; it is consumed before decoding. Rename any such YAML key and
+corresponding Go field, then rebuild. No collection registration or opt-in flag
+is available. Configurations without `extends` retain their existing meaning.
+
+Applications adopting the new features may move source-backed defaults into
+explicit `Underlay` groups, use `SourceContext` for relative resources, or
+replace application-owned sibling merging with one `extends` relationship per
+mapping. Adoption is optional. The maintained
+[named-preset example](../consumers/README.md#named-sibling-inheritance)
+shows inherited `false` and destination-local nested resolution.
+
+Select the new module version:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.5.0
+```
+
+The minimum Go line remains 1.25. Linux amd64 and arm64 remain the
+release-validated targets; macOS remains best-effort. The YAML dependency is
+unchanged at `go.yaml.in/yaml/v4 v4.0.0-rc.6`.
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.5.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.5.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.5.0/docs/oss/attribution.md)

docs/releases/v0.6.0.md

@@ -0,0 +1,63 @@
+# ConfigKit v0.6.0
+
+This minor release reduces avoidable configuration-loading and inspection work,
+adopts ReleaseKit for source publication, and raises the consumer Go minimum to
+1.26.0. The public configuration model remains unchanged from v0.5.0.
+
+## Changes
+
+- Reduce temporary path allocations by sizing pointer-formatting buffers,
+  combining final metadata traversal, reusing parsed sensitivity paths, and
+  avoiding path construction when detecting inheritance. Effective and
+  historical metadata indexes retain independently owned paths.
+- Replace repeated resolution-coverage, metadata-update, CLI-overlap, and
+  sensitivity-propagation scans with private indexes and incremental processing.
+  Preserve diagnostic ordering, history, contextual path shapes, and redaction.
+- Make presence-only metadata queries avoid materializing provenance. Prepare
+  resolution replay descriptors and immutable mapping and inheritance-history
+  lookups for inspection, and use ordered membership indexes for provenance
+  collections. Returned records remain detached; explanations still replay
+  complete trees and render on demand.
+- Add a maintained public-API startup construction benchmark. The initial path
+  formatting and metadata traversal experiments reproduced allocation reductions
+  across local and downstream workloads. Later algorithmic improvements were
+  validated behaviorally without a new benchmark campaign. Busy-host timings
+  were inconclusive; no fixed speedup or allocation threshold is promised.
+- Replace ConfigKit-owned release scripts with a small profile and the
+  module-selected ReleaseKit v0.3.0 tool. The
+  [release procedure](../release.md) documents candidate preparation, tag-only
+  publication, and failed-push recovery. Existing release checks and the
+  successful-tag-push completion point remain unchanged; CI is not a gate.
+- Adopt release policy 1.6, distinguishing the consumer Go minimum from the
+  validation toolchain. Retire completed planning and measurement records into
+  Git history while preserving [deferred work](../roadmap/deferred.md).
+
+## Compatibility and upgrade
+
+The consumer minimum increases from Go 1.25.0 to **Go 1.26.0**. Install Go 1.26.0
+or newer before upgrading, and update application build and CI environments that
+still use Go 1.25. The `go.mod` directive records the consumer minimum; using a
+newer validation patch does not itself raise that minimum. See the
+[consumer minimum policy](../policy/release.md#consumer-minimum-and-release-toolchain).
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.6.0
+```
+
+No application-code or configuration migration is required from v0.5.0. Public
+Go signatures, strict within-tier composition, recursive tier precedence,
+sibling inheritance, presence/null distinctions, provenance, secret handling,
+and synchronous completion retain their existing contracts. See the
+[configuration contract](../config.md) and
+[consumer guide](../consumers/README.md).
+
+The YAML dependency remains `go.yaml.in/yaml/v4 v4.0.0-rc.6`. ReleaseKit is a
+declared Go tool dependency for maintainer commands, not a library runtime
+import; its requirements participate in ConfigKit's module graph. Linux amd64
+and arm64 remain release-validated targets, and macOS remains best-effort.
+Maintainers should use the current release procedure instead of the retired
+`scripts/release.sh` and `scripts/release-check.sh` commands.
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.6.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.6.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.6.0/docs/oss/attribution.md)

docs/releases/v0.7.0.md

@@ -0,0 +1,58 @@
+# ConfigKit v0.7.0
+
+This minor release adds structured comparison of completed configurations.
+Applications can inspect changes between two configuration files or the
+effective impact of a selected overlay, with ConfigKit handling comparison and
+safe value disclosure while applications own presentation.
+
+## Changes
+
+- Add `Diff[T](left, right *Result[T]) ([]Difference, error)` through the public
+  facade. It compares frozen effective inspection values from successful loads,
+  including defaults and recorded application resolution, rather than source
+  text or later mutations of `Result.Config`.
+- Return detached changed-region records containing a semantic path, safe left
+  and right `ValueSnapshot` values, and `HiddenChanged`. Mappings compare
+  recursively, sequences report atomic changes, and records follow canonical
+  pointer order. Presence distinguishes additions and removals from explicit
+  null; equal effective values produce no records regardless of provenance or
+  loading history.
+- Protect both snapshots with sensitivity from both loads, including historical
+  and dependency-derived classification. Compatible regions retain public
+  siblings; incompatible structural replacements use complete-region masking
+  where required. `HiddenChanged` identifies actual changes withheld by masking,
+  rather than merely indicating that a snapshot contains redacted content.
+- Preserve visible public scalar changes when only historical sensitive
+  descendants remain, including additions and removals. Historical explanation
+  redaction and masking of incompatible present shapes remain intact.
+- Add maintained file-comparison and base-versus-overlay examples with
+  consumer-owned formatting, plus an executable public API example. See the
+  [consumer guide](../consumers/README.md#effective-configuration-diffs) and
+  [configuration contract](../config.md#effective-configuration-diffs).
+
+## Compatibility and upgrade
+
+No application-code or configuration migration is required from v0.6.0.
+Adopting the new API is optional. Loading, composition, inheritance, provenance,
+existing inspection, and secret-handling contracts remain unchanged.
+
+Load both configurations successfully before calling `Diff`; nil or
+uninitialized results return an error with no records. Comparison does not
+reopen sources, rerun callbacks, or reread environment variables. Typed secret
+references compare by configured reference name, not resolved credential bytes,
+so credential rotation behind an unchanged name is not reported. Returned
+snapshots describe changed regions, not cross-protected full-document context.
+
+Select this version and rebuild:
+
+```sh
+go get gitea.maximumdirect.net/eric/ConfigKit@v0.7.0
+```
+
+The consumer minimum remains Go 1.26.0. The YAML dependency remains
+`go.yaml.in/yaml/v4 v4.0.0-rc.6`, and the maintainer tool dependency remains
+ReleaseKit v0.3.0. Linux amd64 and arm64 remain release-validated targets;
+macOS remains best-effort. Publication follows the existing
+[release procedure](../release.md).
+
+[COPYRIGHT](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.0/COPYRIGHT) | [LICENSE](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.0/LICENSE) | [ATTRIBUTION](https://gitea.maximumdirect.net/eric/ConfigKit/src/tag/v0.7.0/docs/oss/attribution.md)

docs/roadmap/deferred.md

@@ -11,18 +11,47 @@ General environment-variable configuration overlays are not supported. Typed sec
 references and explicit directory bootstrap are the environment convention. Literal
 credential configuration and implicit secret-directory discovery are unsupported.
 
-The initial release deliberately excludes:
+The following remain deliberately excluded:
 
 - automatic `conf.d` discovery;
 - remote imports, arbitrary providers, and additional configuration formats;
-- custom merge strategies, sequence merging, reset operators, and overlay inheritance;
+- custom merge strategies, sequence merging, reset operators, multiple-parent
+  inheritance, and cross-scope references;
+- named overlay inheritance;
 - parser-specific or custom structural-decoding extension APIs;
 - generic dynamic-subtree conversion and module-registry APIs;
 - automatic schema migrations;
 - hot reload, runtime resource replacement, and remote resource validation;
 - a bundled CLI framework or application commands; and
-- configuration diffs and fingerprints.
+- configuration fingerprints.
 
 Applications may build their own validate, effective-value, source, bootstrap, and
 explanation commands around the library. Broadening these boundaries requires
 concrete consumer evidence, compatibility review, and an explicit design change.
+
+Effective configuration diffs are implemented over frozen successful-load
+inspections; their current comparison and disclosure contract is in the
+[configuration contract](../config.md#effective-configuration-diffs), with
+consumer workflows in the [adoption guide](../consumers/README.md#effective-configuration-diffs).
+Fingerprints remain deferred.
+
+Built-in sibling inheritance using `extends` is implemented as described in the
+[configuration contract](../config.md#sibling-inheritance). It is distinct from
+inheritance between selected overlay files, which remains excluded above.
+
+## Deferred performance experiments
+
+The completed performance audit left these internal optimization candidates for
+separate investigation:
+
+- branch-local explanation replay instead of the current whole-tree replay;
+- redesigning the private metadata-copy handoff while preserving detached public
+  records;
+- caching schema descriptors across loads; and
+- indexing complex contributor-trace membership while preserving its equality
+  rules and compact linked traces.
+
+These are not committed implementation work. Reconsider them with evidence that
+the benefit justifies the ownership, retained-memory, or equivalence risks. The
+completed audit and implementation evidence are preserved in Git history at
+`8e91a2b` under `docs/roadmap/`.

example_test.go

@@ -30,6 +30,32 @@ func ExampleNoSource() {
 	// Output: 4 0 <nil>
 }
 
+func ExampleDiff() {
+	type Config struct {
+		Workers int `yaml:"workers"`
+	}
+	before, err := configkit.Load[Config](configkit.Mapping("before", map[string]any{"workers": 2}))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	after, err := configkit.Load[Config](configkit.Mapping("after", map[string]any{"workers": 4}))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	differences, err := configkit.Diff(before, after)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	for _, difference := range differences {
+		fmt.Printf("%s: %s -> %s\n", difference.Path.String(),
+			strings.TrimSpace(difference.Left.YAML), strings.TrimSpace(difference.Right.YAML))
+	}
+	// Output: /workers: 2 -> 4
+}
+
 func ExampleValidation_Error() {
 	type Config struct {
 		First  string `yaml:"first"`

examples/consumers/consumers_test.go

@@ -52,6 +52,22 @@ func Example_seriatim() {
 	// Output: visitor true the event occurred <nil> <nil>
 }
 
+func Example_inheritedPresets() {
+	result, err := consumers.LoadPresets(configkit.File(fixture("inheritance", "presets.yml")))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	path := configkit.Keys("presets", "custom", "enabled")
+	info, _ := result.Metadata().Lookup(path)
+	custom := result.Config.Presets["custom"]
+	fmt.Println(custom.Enabled, result.Metadata().IsExplicit(path), custom.Settings["secondary"]["width"])
+	fmt.Println(info.Origin.Locations[0].SourceID, info.Origin.Inheritance[0].From)
+	// Output:
+	// false true 20
+	// source-1 /presets/base/enabled
+}
+
 func Example_weatherStartup() {
 	const blockedName = "CONFIGKIT_WEATHER_EXAMPLE_BLOCKED"
 	const tokenName = "CONFIGKIT_WEATHER_EXAMPLE_TOKEN"

examples/consumers/diff.go

@@ -0,0 +1,136 @@
+package consumers
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+// FileComparisonConfig is a small application-owned shape for comparing
+// caller-selected configuration files.
+type FileComparisonConfig struct {
+	Name    string         `yaml:"name"`
+	Entries map[string]any `yaml:"entries"`
+}
+
+// CompareFiles loads two caller-selected files with the same application
+// definition, then formats only the safe records returned by configkit.Diff.
+func CompareFiles(leftPath, rightPath string) (string, error) {
+	options := configkit.Options[FileComparisonConfig]{
+		Definition: configkit.Definition[FileComparisonConfig]{
+			Defaults: func() FileComparisonConfig {
+				return FileComparisonConfig{Entries: map[string]any{}}
+			},
+		},
+	}
+	left, err := configkit.Load(configkit.File(leftPath), options)
+	if err != nil {
+		return "", fmt.Errorf("load left configuration: %w", err)
+	}
+	right, err := configkit.Load(configkit.File(rightPath), options)
+	if err != nil {
+		return "", fmt.Errorf("load right configuration: %w", err)
+	}
+	differences, err := configkit.Diff(left, right)
+	if err != nil {
+		return "", err
+	}
+	return FormatDifferences(differences), nil
+}
+
+// RolloutConfig models a service endpoint whose URL is derived by the
+// application resolver and whose credential reference is classified.
+type RolloutConfig struct {
+	Service RolloutService `yaml:"service"`
+}
+
+// RolloutService contains endpoint settings and a classified credential.
+type RolloutService struct {
+	Endpoint   RolloutEndpoint   `yaml:"endpoint"`
+	Credential RolloutCredential `yaml:"credential"`
+}
+
+// RolloutEndpoint is the effective destination used by the application.
+type RolloutEndpoint struct {
+	Host string `yaml:"host"`
+	Port int    `yaml:"port"`
+	URL  string `yaml:"url"`
+}
+
+// RolloutCredential holds only a typed secret reference in configuration.
+type RolloutCredential struct {
+	Token configkit.Secret `yaml:"token" configkit:"secret"`
+}
+
+// LoadRollout loads a base file, optionally with one selected configuration
+// overlay. Resolution derives the effective URL after the overlay is applied.
+func LoadRollout(basePath, overlayName string) (*configkit.Result[RolloutConfig], error) {
+	return configkit.Load(configkit.File(basePath), configkit.Options[RolloutConfig]{
+		Overlay: overlayName,
+		Definition: configkit.Definition[RolloutConfig]{
+			Resolve: resolveRollout,
+		},
+	})
+}
+
+// CompareRolloutOverlay compares the effective base configuration with that
+// same file plus one selected overlay. It presents only safe diff records.
+func CompareRolloutOverlay(basePath, overlayName string) (string, error) {
+	base, err := LoadRollout(basePath, "")
+	if err != nil {
+		return "", fmt.Errorf("load base configuration: %w", err)
+	}
+	withOverlay, err := LoadRollout(basePath, overlayName)
+	if err != nil {
+		return "", fmt.Errorf("load overlay configuration: %w", err)
+	}
+	differences, err := configkit.Diff(base, withOverlay)
+	if err != nil {
+		return "", err
+	}
+	return FormatDifferences(differences), nil
+}
+
+func resolveRollout(configuration *RolloutConfig, resolution *configkit.Resolution) error {
+	endpoint := &configuration.Service.Endpoint
+	if endpoint.Host == "" || endpoint.Port <= 0 {
+		return nil
+	}
+	url := fmt.Sprintf("https://%s:%d", endpoint.Host, endpoint.Port)
+	if endpoint.URL != url {
+		endpoint.URL = url
+		resolution.Record(configkit.Keys("service", "endpoint", "url"),
+			configkit.ChangeNormalization, "derived endpoint URL",
+			configkit.Keys("service", "endpoint", "host"),
+			configkit.Keys("service", "endpoint", "port"))
+	}
+	return nil
+}
+
+// FormatDifferences is consumer-owned presentation. It distinguishes absence
+// through Present and reports withheld changes through HiddenChanged without
+// interpreting redaction marker text.
+func FormatDifferences(differences []configkit.Difference) string {
+	if len(differences) == 0 {
+		return "No effective changes.\n"
+	}
+	var output strings.Builder
+	for _, difference := range differences {
+		fmt.Fprintf(&output, "%s: left=%s right=%s", difference.Path.String(),
+			formatSnapshot(difference.Left), formatSnapshot(difference.Right))
+		if difference.HiddenChanged {
+			output.WriteString(" hidden-change")
+		}
+		output.WriteByte('\n')
+	}
+	return output.String()
+}
+
+func formatSnapshot(snapshot configkit.ValueSnapshot) string {
+	if !snapshot.Present {
+		return "<absent>"
+	}
+	return strconv.Quote(snapshot.YAML)
+}

examples/consumers/diff_test.go

@@ -0,0 +1,187 @@
+package consumers_test
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func ExampleCompareFiles() {
+	directory, err := os.MkdirTemp("", "configkit-diff-files-")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer os.RemoveAll(directory)
+	leftPath := filepath.Join(directory, "before.yml")
+	rightPath := filepath.Join(directory, "after.yml")
+	if err := os.WriteFile(leftPath, []byte("name: before\nentries: {}\n"), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	if err := os.WriteFile(rightPath, []byte("name: after\nentries:\n  added: null\n"), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	output, err := consumers.CompareFiles(leftPath, rightPath)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Print(output)
+	// Output:
+	// /entries/added: left=<absent> right="null\n"
+	// /name: left="before\n" right="after\n"
+}
+
+func ExampleCompareRolloutOverlay() {
+	const baseToken = "CONFIGKIT_DIFF_EXAMPLE_BASE_TOKEN"
+	const overlayToken = "CONFIGKIT_DIFF_EXAMPLE_OVERLAY_TOKEN"
+	defer preserveEnvironment(baseToken)()
+	defer preserveEnvironment(overlayToken)()
+	_ = os.Setenv(baseToken, "disposable-example-base-credential")
+	_ = os.Setenv(overlayToken, "disposable-example-overlay-credential")
+
+	directory, err := os.MkdirTemp("", "configkit-diff-overlay-")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer os.RemoveAll(directory)
+	basePath := filepath.Join(directory, "config.yml")
+	overlayPath := filepath.Join(directory, "overlays", "canary.yml")
+	if err := os.MkdirAll(filepath.Dir(overlayPath), 0o700); err != nil {
+		fmt.Println(err)
+		return
+	}
+	base := "service:\n  endpoint:\n    host: base.example\n    port: 8443\n  credential:\n    token: " + baseToken + "\n"
+	overlay := "service:\n  endpoint:\n    host: canary.example\n  credential:\n    token: " + overlayToken + "\n"
+	if err := os.WriteFile(basePath, []byte(base), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	if err := os.WriteFile(overlayPath, []byte(overlay), 0o600); err != nil {
+		fmt.Println(err)
+		return
+	}
+	output, err := consumers.CompareRolloutOverlay(basePath, "canary")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Print(output)
+	// Output:
+	// /service/credential/token: left="'[REDACTED]'\n" right="'[REDACTED]'\n" hidden-change
+	// /service/endpoint/host: left="base.example\n" right="canary.example\n"
+	// /service/endpoint/url: left="https://base.example:8443\n" right="https://canary.example:8443\n"
+}
+
+func TestCompareFilesUsesCallerSelectedSourcesAndSafePresence(t *testing.T) {
+	directory := t.TempDir()
+	leftPath := filepath.Join(directory, "before.yml")
+	rightPath := filepath.Join(directory, "after.yml")
+	writeConfiguration(t, leftPath, "name: before\nentries: {}\n")
+	writeConfiguration(t, rightPath, "name: after\nentries:\n  added: null\n")
+
+	output, err := consumers.CompareFiles(leftPath, rightPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "/entries/added: left=<absent> right=\"null\\n\"\n" +
+		"/name: left=\"before\\n\" right=\"after\\n\"\n"
+	if output != want {
+		t.Fatalf("comparison output = %q, want %q", output, want)
+	}
+	if again, err := consumers.CompareFiles(leftPath, rightPath); err != nil || again != output {
+		t.Fatalf("repeat output = %q, %v", again, err)
+	}
+}
+
+func TestCompareFilesReturnsEachLoadError(t *testing.T) {
+	directory := t.TempDir()
+	leftPath := filepath.Join(directory, "before.yml")
+	rightPath := filepath.Join(directory, "after.yml")
+	writeConfiguration(t, leftPath, "name: before\n")
+	writeConfiguration(t, rightPath, "unknown: invalid\n")
+
+	output, err := consumers.CompareFiles(leftPath, rightPath)
+	if err == nil || output != "" || !strings.Contains(err.Error(), "load right configuration") {
+		t.Fatalf("right load failure = %q, %v", output, err)
+	}
+	if _, err := consumers.CompareFiles(filepath.Join(directory, "missing.yml"), rightPath); err == nil || !strings.Contains(err.Error(), "load left configuration") {
+		t.Fatalf("left load failure = %v", err)
+	}
+}
+
+func TestCompareRolloutOverlayShowsResolvedSafeChangesAndRetainsSiblings(t *testing.T) {
+	t.Setenv("CONFIGKIT_DIFF_BASE_TOKEN", "disposable-base-credential")
+	t.Setenv("CONFIGKIT_DIFF_OVERLAY_TOKEN", "disposable-overlay-credential")
+	directory := t.TempDir()
+	basePath := filepath.Join(directory, "config.yml")
+	overlayPath := filepath.Join(directory, "overlays", "canary.yml")
+	if err := os.MkdirAll(filepath.Dir(overlayPath), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	writeConfiguration(t, basePath, "service:\n  endpoint:\n    host: base.example\n    port: 8443\n  credential:\n    token: CONFIGKIT_DIFF_BASE_TOKEN\n")
+	writeConfiguration(t, overlayPath, "service:\n  endpoint:\n    host: canary.example\n  credential:\n    token: CONFIGKIT_DIFF_OVERLAY_TOKEN\n")
+
+	base, err := consumers.LoadRollout(basePath, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	withOverlay, err := consumers.LoadRollout(basePath, "canary")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if base.Config.Service.Endpoint.Port != 8443 || withOverlay.Config.Service.Endpoint.Port != 8443 {
+		t.Fatalf("lower-priority port was not retained: base=%#v overlay=%#v", base.Config.Service.Endpoint, withOverlay.Config.Service.Endpoint)
+	}
+	urlInfo, found := withOverlay.Metadata().Lookup(configkit.Keys("service", "endpoint", "url"))
+	if !found || urlInfo.Origin.Layer != configkit.LayerResolution || withOverlay.Config.Service.Endpoint.URL != "https://canary.example:8443" {
+		t.Fatalf("resolved endpoint = %#v, metadata=%#v, %v", withOverlay.Config.Service.Endpoint, urlInfo, found)
+	}
+
+	differences, err := configkit.Diff(base, withOverlay)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(differences) != 3 {
+		t.Fatalf("differences = %#v", differences)
+	}
+	wantPaths := []string{"/service/credential/token", "/service/endpoint/host", "/service/endpoint/url"}
+	for index, difference := range differences {
+		if got := difference.Path.String(); got != wantPaths[index] {
+			t.Errorf("path[%d] = %q, want %q", index, got, wantPaths[index])
+		}
+	}
+	secret := differences[0]
+	if !secret.Left.Redacted || !secret.Right.Redacted || !secret.HiddenChanged || secret.Left.YAML != "'[REDACTED]'\n" || secret.Right.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("classified credential difference = %#v", secret)
+	}
+	if strings.Contains(consumers.FormatDifferences(differences), "disposable-") {
+		t.Fatal("formatted differences exposed credential contents")
+	}
+
+	output, err := consumers.CompareRolloutOverlay(basePath, "canary")
+	if err != nil || output != consumers.FormatDifferences(differences) {
+		t.Fatalf("formatted base/overlay comparison = %q, %v", output, err)
+	}
+}
+
+func TestFormatDifferencesUsesRecordMembershipForNoChange(t *testing.T) {
+	if got := consumers.FormatDifferences(nil); got != "No effective changes.\n" {
+		t.Fatalf("no-change output = %q", got)
+	}
+}
+
+func writeConfiguration(t *testing.T, filename, contents string) {
+	t.Helper()
+	if err := os.WriteFile(filename, []byte(contents), 0o600); err != nil {
+		t.Fatal(err)
+	}
+}

examples/consumers/inheritance.go

@@ -0,0 +1,20 @@
+package consumers
+
+import configkit "gitea.maximumdirect.net/eric/ConfigKit"
+
+// PresetCatalog is ordinary application data; no inheritance fields or
+// registration hooks are required in the typed configuration.
+type PresetCatalog struct {
+	Presets map[string]Preset `yaml:"presets"`
+}
+
+type Preset struct {
+	Enabled  bool                      `yaml:"enabled"`
+	Settings map[string]map[string]int `yaml:"settings"`
+}
+
+// LoadPresets performs one load; callers may inspect inherited presence and
+// source origins on the returned result before choosing a preset to execute.
+func LoadPresets(source configkit.Source) (*configkit.Result[PresetCatalog], error) {
+	return configkit.Load[PresetCatalog](source)
+}

examples/consumers/inheritance_test.go

@@ -0,0 +1,35 @@
+package consumers_test
+
+import (
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func TestPresetCatalogOneLoadAndDerivation(t *testing.T) {
+	result, err := consumers.LoadPresets(configkit.File(fixture("inheritance", "presets.yml")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.Config.Presets["base"].Settings["secondary"]["width"] != 10 ||
+		result.Config.Presets["custom"].Settings["secondary"]["width"] != 20 ||
+		result.Config.Presets["custom"].Enabled {
+		t.Fatalf("expanded presets = %#v", result.Config.Presets)
+	}
+	enabled := configkit.Keys("presets", "custom", "enabled")
+	if !result.Metadata().IsExplicit(enabled) {
+		t.Fatal("inherited false lost explicit presence")
+	}
+	info, found := result.Metadata().Lookup(enabled)
+	if !found || info.Origin.Locations[0].SourceID != "source-1" ||
+		len(info.Origin.Inheritance) != 1 || info.Origin.Inheritance[0].From.String() != "/presets/base/enabled" {
+		t.Fatalf("origin and derivation = %#v, %v", info, found)
+	}
+	nested := configkit.Keys("presets", "custom", "settings", "secondary", "width")
+	info, found = result.Metadata().Lookup(nested)
+	if !found || len(info.Origin.Inheritance) != 1 ||
+		info.Origin.Inheritance[0].From.String() != "/presets/custom/settings/primary/width" {
+		t.Fatalf("destination-local derivation = %#v, %v", info, found)
+	}
+}

examples/consumers/layered.go

@@ -0,0 +1,95 @@
+package consumers
+
+import (
+	"embed"
+	"errors"
+	"io/fs"
+	"path"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+//go:embed testdata/native-source testdata/layered-bundle
+var bundledFiles embed.FS
+
+// BundledConfig is a small configuration loaded directly from an embedded file.
+type BundledConfig struct {
+	Name    string `yaml:"name"`
+	Enabled bool   `yaml:"enabled"`
+}
+
+// LoadBundled demonstrates a native filesystem source without layered options.
+func LoadBundled() (*configkit.Result[BundledConfig], error) {
+	files, err := fs.Sub(bundledFiles, "testdata/native-source")
+	if err != nil {
+		return nil, err
+	}
+	return configkit.Load[BundledConfig](configkit.NewFilesystem("bundled", files).File("config.yml"))
+}
+
+// CatalogConfig keeps resource names and domain rules in the application.
+type CatalogConfig struct {
+	Mode      string            `yaml:"mode"`
+	Enabled   bool              `yaml:"enabled"`
+	Workers   int               `yaml:"workers"`
+	Templates map[string]string `yaml:"templates"`
+	Labels    map[string]string `yaml:"labels"`
+}
+
+// LoadCatalog combines two underlay tiers beneath an independently located
+// host main file. ConfigKit assembles and validates the complete result once.
+func LoadCatalog(mainPath string) (*configkit.Result[CatalogConfig], error) {
+	files, err := fs.Sub(bundledFiles, "testdata/layered-bundle")
+	if err != nil {
+		return nil, err
+	}
+	bundle := configkit.NewFilesystem("catalog bundle", files)
+	return configkit.LoadLayered(configkit.Input{Source: configkit.File(mainPath)},
+		configkit.LayeredOptions[CatalogConfig]{
+			Underlays: []configkit.Underlay{
+				{Inputs: []configkit.Input{{Source: bundle.File("defaults/config.yml"), Imports: true}}},
+				{Inputs: []configkit.Input{{Source: configkit.Mapping("local policy", map[string]any{
+					"workers": 2,
+					"labels":  map[string]any{"tenant": "example"},
+				})}}},
+			},
+			Definition: configkit.Definition[CatalogConfig]{
+				Defaults: func() CatalogConfig { return CatalogConfig{Mode: "demo"} },
+				Validate: validateCatalog,
+			},
+		})
+}
+
+func validateCatalog(configuration *CatalogConfig, validation *configkit.Validation) {
+	if configuration.Mode != "demo" && configuration.Mode != "production" {
+		validation.Error(configkit.Keys("mode"), "must be demo or production")
+	}
+	if configuration.Workers < 0 {
+		validation.Error(configkit.Keys("workers"), "must be nonnegative")
+	}
+	if name := configuration.Templates["welcome"]; name == "." || !fs.ValidPath(name) {
+		validation.Error(configkit.Keys("templates", "welcome"), "must be a relative resource name")
+	}
+}
+
+// ReadWelcome follows the effective template field's one declaring source to
+// its retained resource context. Opening and interpreting the resource remain
+// application work; this does not reopen configuration or change metadata.
+func ReadWelcome(result *configkit.Result[CatalogConfig]) (string, error) {
+	if result == nil {
+		return "", errors.New("catalog result is nil")
+	}
+	info, found := result.Metadata().Lookup(configkit.Keys("templates", "welcome"))
+	if !found || len(info.Origin.Locations) != 1 {
+		return "", errors.New("welcome template has no single declaring source")
+	}
+	context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+	if !found {
+		return "", errors.New("welcome template has no resource context")
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, result.Config.Templates["welcome"]))
+	if err != nil {
+		return "", err
+	}
+	return string(data), nil
+}

examples/consumers/layered_test.go

@@ -0,0 +1,71 @@
+package consumers_test
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+	"gitea.maximumdirect.net/eric/ConfigKit/examples/consumers"
+)
+
+func Example_nativeFilesystem() {
+	result, err := consumers.LoadBundled()
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	fmt.Println(result.Config.Name, result.Config.Enabled)
+	// Output: embedded false
+}
+
+func Example_layeredCatalog() {
+	result, err := consumers.LoadCatalog(fixture("layered-main", "config.yml"))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	welcome, err := consumers.ReadWelcome(result)
+	fmt.Println(result.Config.Mode, result.Config.Enabled, result.Config.Workers,
+		result.Config.Labels["origin"], result.Config.Labels["tenant"], strings.TrimSpace(welcome), err)
+	// Output: production false 0 bundle example Welcome from the bundle. <nil>
+}
+
+func TestCatalogRetainsPresenceAndSourceContext(t *testing.T) {
+	result, err := consumers.LoadCatalog(fixture("layered-main", "config.yml"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, key := range []string{"enabled", "workers"} {
+		info, found := result.Metadata().Lookup(configkit.Keys(key))
+		if !found || !info.Explicit || info.Origin.Layer != configkit.LayerBase {
+			t.Fatalf("%s presence = %#v, %v", key, info, found)
+		}
+	}
+	info, found := result.Metadata().Lookup(configkit.Keys("templates", "welcome"))
+	if !found || info.Origin.Layer != configkit.LayerUnderlay || len(info.Origin.Locations) != 1 {
+		t.Fatalf("welcome origin = %#v, %v", info, found)
+	}
+	context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+	if !found || context.BaseDir != "defaults" {
+		t.Fatalf("welcome context = %#v, %v", context, found)
+	}
+	welcome, err := consumers.ReadWelcome(result)
+	if err != nil || strings.TrimSpace(welcome) != "Welcome from the bundle." {
+		t.Fatalf("welcome = %q, %v", welcome, err)
+	}
+	if result.Config.Labels["tenant"] != "example" || result.Config.Labels["origin"] != "bundle" {
+		t.Fatalf("recursive underlay labels = %#v", result.Config.Labels)
+	}
+}
+
+func TestCatalogValidationRejectsInvalidMode(t *testing.T) {
+	_, err := consumers.LoadCatalog(fixture("layered-main", "invalid.yml"))
+	var loadError *configkit.LoadError
+	if !errors.As(err, &loadError) || len(loadError.Diagnostics()) == 0 ||
+		loadError.Diagnostics()[0].Code != configkit.CodeValidateError ||
+		loadError.Diagnostics()[0].Path.String() != "/mode" {
+		t.Fatalf("invalid mode error = %v", err)
+	}
+}

examples/consumers/testdata/inheritance/presets.yml

@@ -0,0 +1,10 @@
+presets:
+  base:
+    enabled: false
+    settings:
+      primary: {width: 10}
+      secondary: {extends: primary}
+  custom:
+    extends: base
+    settings:
+      primary: {width: 20}

examples/consumers/testdata/layered-bundle/defaults/config.yml

@@ -0,0 +1,7 @@
+configkit:
+  imports:
+    - path: ../shared.yml
+enabled: true
+workers: 4
+templates:
+  welcome: messages/welcome.txt

examples/consumers/testdata/layered-bundle/defaults/messages/welcome.txt

@@ -0,0 +1 @@
+Welcome from the bundle.

examples/consumers/testdata/layered-bundle/shared.yml

@@ -0,0 +1,2 @@
+labels:
+  origin: bundle

examples/consumers/testdata/layered-main/config.yml

@@ -0,0 +1,3 @@
+mode: production
+enabled: false
+workers: 0

examples/consumers/testdata/layered-main/invalid.yml

@@ -0,0 +1 @@
+mode: unsupported

examples/consumers/testdata/native-source/config.yml

@@ -0,0 +1,2 @@
+name: embedded
+enabled: false

facade_candidate_test.go

@@ -31,6 +31,9 @@ func TestFacadeValuesRemainRootOwned(t *testing.T) {
 		reflect.TypeFor[configkit.Resolution](), reflect.TypeFor[configkit.Validation](),
 		reflect.TypeFor[configkit.Version](), reflect.TypeFor[configkit.VersionRule](),
 		reflect.TypeFor[configkit.FilesystemContext](), reflect.TypeFor[configkit.Override](),
+		reflect.TypeFor[configkit.Filesystem](), reflect.TypeFor[configkit.ResourceContext](),
+		reflect.TypeFor[configkit.Input](), reflect.TypeFor[configkit.Underlay](),
+		reflect.TypeFor[configkit.LayeredOptions[struct{}]](),
 		reflect.TypeFor[configkit.Definition[struct{}]](), reflect.TypeFor[configkit.Options[struct{}]](),
 		reflect.TypeFor[configkit.Result[struct{}]](),
 		reflect.TypeFor[configkit.ValueSnapshot](), reflect.TypeFor[configkit.ValueChange](),

facade_context_test.go

@@ -26,6 +26,7 @@ func TestFacadeInactiveContextsPreserveClosedFailure(t *testing.T) {
 				{"IsExplicit", func() { state.resolution.IsExplicit(path) }},
 				{"InputInfo", func() { state.resolution.InputInfo(path) }},
 				{"Source", func() { state.resolution.Source("source") }},
+				{"SourceContext", func() { state.resolution.SourceContext("source") }},
 				{"Record", func() { state.resolution.Record(path, configkit.ChangeNormalization, "reason") }},
 				{"Resolution.Error", func() { state.resolution.Error(path, "message") }},
 				{"Resolution.Warning", func() { state.resolution.Warning(path, "message") }},

facade_diff_test.go

@@ -0,0 +1,327 @@
+package configkit_test
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type facadeDiffConfig struct {
+	Value   string         `yaml:"value"`
+	Count   int            `yaml:"count"`
+	Items   []string       `yaml:"items"`
+	Dynamic map[string]any `yaml:"dynamic"`
+}
+
+type facadeDiffSecretConfig struct {
+	Secret configkit.Secret `yaml:"secret"`
+}
+
+func TestFacadeDiffKeepsPublicSequenceChangesWithHistoricalDescendants(t *testing.T) {
+	type configuration struct {
+		Items []any `yaml:"items"`
+	}
+	load := func(public string) *configkit.Result[configuration] {
+		t.Helper()
+		result, err := configkit.Load(configkit.Bytes("historical-sequence.yml", []byte("items: [{secret: disposable-canary}, "+public+"]\n")),
+			configkit.Options[configuration]{Definition: configkit.Definition[configuration]{
+				Secret: func(path configkit.Path) bool { return path.String() == "/items/0/secret" },
+				Resolve: func(current *configuration, resolution *configkit.Resolution) error {
+					current.Items[0] = "public scalar"
+					resolution.Record(configkit.Keys("items"), configkit.ChangeNormalization, "normalize first item")
+					return nil
+				},
+			}})
+		if err != nil {
+			t.Fatal(err)
+		}
+		return result
+	}
+	left, right := load("1"), load("2")
+	history, ok := left.Explain(configkit.Keys("items").Index(0).Key("secret"))
+	if !ok || history.Effective.Present || len(history.History) == 0 {
+		t.Fatalf("removed historical secret = %#v, known=%v", history, ok)
+	}
+	maskedHistory := false
+	for _, change := range history.History {
+		maskedHistory = maskedHistory || change.Before.Redacted || change.After.Redacted
+	}
+	if !maskedHistory {
+		t.Fatal("historical sensitivity was not retained")
+	}
+	differences, err := configkit.Diff(left, right)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/items" {
+		t.Fatalf("sequence difference = %#v, %v", differences, err)
+	}
+	difference := differences[0]
+	if !difference.Left.Present || !difference.Right.Present || difference.Left.Redacted || difference.Right.Redacted || difference.HiddenChanged {
+		t.Fatalf("public sequence change was concealed: %#v", difference)
+	}
+	if difference.Left.YAML != "- public scalar\n- 1\n" || difference.Right.YAML != "- public scalar\n- 2\n" {
+		t.Fatalf("public sequence snapshots = %#v", difference)
+	}
+}
+
+func TestFacadeDiffKeepsPublicScalarAcrossHistoricalAbsence(t *testing.T) {
+	type configuration struct {
+		Dynamic map[string]any `yaml:"dynamic"`
+	}
+	options := configkit.Options[configuration]{Definition: configkit.Definition[configuration]{
+		Secret: func(path configkit.Path) bool { return path.String() == "/dynamic/entry/token" },
+		Resolve: func(current *configuration, resolution *configkit.Resolution) error {
+			delete(current.Dynamic, "entry")
+			resolution.Record(configkit.Keys("dynamic", "entry"), configkit.ChangeNormalization, "remove entry")
+			return nil
+		},
+	}}
+	absent, err := configkit.Load(configkit.Bytes("removed-entry.yml", []byte("dynamic: {entry: {token: disposable-canary}}\n")), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	options.Definition.Resolve = nil
+	present, err := configkit.Load(configkit.Bytes("public-entry.yml", []byte("dynamic: {entry: public}\n")), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, test := range []struct {
+		name        string
+		left, right *configkit.Result[configuration]
+		leftPresent bool
+	}{
+		{name: "addition", left: absent, right: present},
+		{name: "removal", left: present, right: absent, leftPresent: true},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			differences, err := configkit.Diff(test.left, test.right)
+			if err != nil || len(differences) != 1 || differences[0].Path.String() != "/dynamic/entry" {
+				t.Fatalf("entry difference = %#v, %v", differences, err)
+			}
+			difference := differences[0]
+			if difference.Left.Present != test.leftPresent || difference.Right.Present == test.leftPresent ||
+				difference.Left.Redacted || difference.Right.Redacted || difference.HiddenChanged {
+				t.Fatalf("public presence change was concealed: %#v", difference)
+			}
+			wantLeft, wantRight := "", "public\n"
+			if test.leftPresent {
+				wantLeft, wantRight = wantRight, wantLeft
+			}
+			if difference.Left.YAML != wantLeft || difference.Right.YAML != wantRight {
+				t.Fatalf("entry snapshots = %#v", difference)
+			}
+		})
+	}
+}
+
+func TestFacadeDiffReturnsDetachedCanonicalRecords(t *testing.T) {
+	left, err := configkit.Load[facadeDiffConfig](configkit.Bytes("left.yml", []byte("value: before\ncount: 0\nitems: [one, two]\ndynamic: {}\n")))
+	if err != nil {
+		t.Fatalf("load left: %v", err)
+	}
+	right, err := configkit.Load[facadeDiffConfig](configkit.Bytes("right.yml", []byte("value: after\ncount: 0\nitems: [one, three]\ndynamic:\n  added: null\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	differences, err := configkit.Diff(left, right)
+	if err != nil {
+		t.Fatal(err)
+	}
+	wantPaths := []string{"/dynamic/added", "/items", "/value"}
+	if len(differences) != len(wantPaths) {
+		t.Fatalf("differences = %#v", differences)
+	}
+	for index, difference := range differences {
+		if got := difference.Path.String(); got != wantPaths[index] {
+			t.Errorf("path[%d] = %q, want %q", index, got, wantPaths[index])
+		}
+	}
+	if differences[0].Left.Present || !differences[0].Right.Present || differences[0].Right.YAML != "null\n" {
+		t.Errorf("null/absence snapshots = %#v / %#v", differences[0].Left, differences[0].Right)
+	}
+	if differences[1].Left.YAML != "- one\n- two\n" || differences[1].Right.YAML != "- one\n- three\n" {
+		t.Errorf("sequence snapshots = %#v / %#v", differences[1].Left, differences[1].Right)
+	}
+
+	left.Config.Value = "caller mutation"
+	left.Config.Items[0] = "caller mutation"
+	right.Config.Dynamic["added"] = "caller mutation"
+	differences[2].Left.YAML = "record mutation"
+	differences[2].Path = configkit.Keys("rewritten")
+
+	again, err := configkit.Diff(left, right)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if again[2].Path.String() != "/value" || again[2].Left.YAML != "before\n" || again[1].Left.YAML != "- one\n- two\n" {
+		t.Fatalf("repeat diff changed = %#v", again)
+	}
+	explanation, ok := left.Explain(configkit.Keys("value"))
+	if !ok || explanation.Effective.YAML != "before\n" {
+		t.Fatalf("frozen explanation = %#v, %v", explanation, ok)
+	}
+}
+
+func TestFacadeDiffRejectsUnavailableResults(t *testing.T) {
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	loaded, err := configkit.Load[config](configkit.Bytes("valid.yml", []byte("value: valid\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var nilResult *configkit.Result[config]
+	zeroResult := &configkit.Result[config]{Config: config{Value: "must not be inspected"}}
+	for name, left := range map[string]*configkit.Result[config]{"nil": nilResult, "zero": zeroResult} {
+		t.Run(name, func(t *testing.T) {
+			if records, err := configkit.Diff(left, loaded); err == nil || records != nil {
+				t.Fatalf("Diff = %#v, %v; want nil records and error", records, err)
+			}
+		})
+	}
+	if records, err := configkit.Diff(loaded, nilResult); err == nil || records != nil {
+		t.Fatalf("right nil Diff = %#v, %v", records, err)
+	}
+}
+
+func TestFacadeDiffIgnoresSourceAndResolutionHistory(t *testing.T) {
+	type config struct {
+		Count int `yaml:"count"`
+	}
+	defaults := configkit.Options[config]{Definition: configkit.Definition[config]{
+		Defaults: func() config { return config{Count: 7} },
+	}}
+	defaulted, err := configkit.Load[config](configkit.NoSource(), defaults)
+	if err != nil {
+		t.Fatal(err)
+	}
+	explicit, err := configkit.Load[config](configkit.Bytes("explicit.yml", []byte("count: 7\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(defaulted, explicit); err != nil || len(differences) != 0 {
+		t.Fatalf("default/source difference = %#v, %v", differences, err)
+	}
+
+	zeroDefault, err := configkit.Load[config](configkit.NoSource(), configkit.Options[config]{Definition: configkit.Definition[config]{Defaults: func() config { return config{} }}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	explicitZero, err := configkit.Load[config](configkit.Bytes("explicit-zero.yml", []byte("count: 0\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(zeroDefault, explicitZero); err != nil || len(differences) != 0 {
+		t.Fatalf("explicit/defaulted zero difference = %#v, %v", differences, err)
+	}
+
+	resolve := func(value string) configkit.Options[config] {
+		return configkit.Options[config]{Definition: configkit.Definition[config]{Resolve: func(current *config, resolution *configkit.Resolution) error {
+			current.Count = len(value) + 8
+			resolution.Record(configkit.Keys("count"), configkit.ChangeNormalization, "set effective count")
+			return nil
+		}}}
+	}
+	resolvedA, err := configkit.Load[config](configkit.Bytes("a.yml", []byte("{}\n")), resolve("a"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	resolvedB, err := configkit.Load[config](configkit.Bytes("b.yml", []byte("{}\n")), resolve("b"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(resolvedA, resolvedB); err != nil || len(differences) != 0 {
+		t.Fatalf("equal resolved values = %#v, %v", differences, err)
+	}
+}
+
+func TestFacadeDiffUsesFrozenFilesAndRunsCallbacksOnlyDuringLoad(t *testing.T) {
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	calls := 0
+	options := configkit.Options[config]{Definition: configkit.Definition[config]{Defaults: func() config {
+		calls++
+		return config{}
+	}}}
+	dir := t.TempDir()
+	paths := []string{filepath.Join(dir, "left.yml"), filepath.Join(dir, "right.yml")}
+	for index, contents := range []string{"value: left\n", "value: right\n"} {
+		if err := os.WriteFile(paths[index], []byte(contents), 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	left, err := configkit.Load[config](configkit.File(paths[0]), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	right, err := configkit.Load[config](configkit.File(paths[1]), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if calls != 2 {
+		t.Fatalf("defaults calls after loads = %d", calls)
+	}
+	for _, path := range paths {
+		if err := os.Remove(path); err != nil {
+			t.Fatal(err)
+		}
+	}
+	differences, err := configkit.Diff(left, right)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/value" {
+		t.Fatalf("Diff after removing files = %#v, %v", differences, err)
+	}
+	if calls != 2 {
+		t.Fatalf("defaults reran during Diff: %d calls", calls)
+	}
+}
+
+func TestFacadeDiffCombinesSecretSensitivityAndLoadErrors(t *testing.T) {
+	t.Setenv("FACADE_DIFF_SECRET", "credential-one")
+	secretOptions := configkit.Options[facadeDiffSecretConfig]{Definition: configkit.Definition[facadeDiffSecretConfig]{Secret: func(path configkit.Path) bool {
+		return path.String() == "/secret"
+	}}}
+	left, err := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("left.yml", []byte("secret: FACADE_DIFF_SECRET\n")), secretOptions)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("FACADE_DIFF_SECRET", "credential-two")
+	right, err := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("right.yml", []byte("secret: FACADE_DIFF_SECRET\n")), secretOptions)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if differences, err := configkit.Diff(left, right); err != nil || len(differences) != 0 {
+		t.Fatalf("credential rotation = %#v, %v", differences, err)
+	}
+	t.Setenv("OTHER_FACADE_DIFF_SECRET", "other-credential")
+	otherReference, err := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("other.yml", []byte("secret: OTHER_FACADE_DIFF_SECRET\n")), secretOptions)
+	if err != nil {
+		t.Fatal(err)
+	}
+	differences, err := configkit.Diff(left, otherReference)
+	if err != nil || len(differences) != 1 || !differences[0].HiddenChanged || !differences[0].Left.Redacted || !differences[0].Right.Redacted {
+		t.Fatalf("reference change = %#v, %v", differences, err)
+	}
+	if strings.Contains(differences[0].Left.YAML+differences[0].Right.YAML, "credential") {
+		t.Fatalf("credential leaked: %#v", differences)
+	}
+
+	invalidLeft, leftErr := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("invalid-left.yml", []byte("unknown: true\n")))
+	if invalidLeft != nil || leftErr == nil {
+		t.Fatalf("invalid left load = %#v, %v", invalidLeft, leftErr)
+	}
+	var loadError *configkit.LoadError
+	if !errors.As(leftErr, &loadError) {
+		t.Fatalf("left load error = %T, want *configkit.LoadError", leftErr)
+	}
+	invalidRight, rightErr := configkit.Load[facadeDiffSecretConfig](configkit.Bytes("invalid-right.yml", []byte("unknown: true\n")))
+	if invalidRight != nil || rightErr == nil {
+		t.Fatalf("invalid right load = %#v, %v", invalidRight, rightErr)
+	}
+	if !errors.As(rightErr, &loadError) {
+		t.Fatalf("right load error = %T, want *configkit.LoadError", rightErr)
+	}
+}

facade_equivalence_test.go

@@ -0,0 +1,164 @@
+package configkit_test
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type equivalentConfig struct {
+	Version  uint64   `yaml:"version"`
+	Name     string   `yaml:"name"`
+	Enabled  bool     `yaml:"enabled"`
+	Count    int      `yaml:"count"`
+	Items    []string `yaml:"items"`
+	Optional *string  `yaml:"optional"`
+}
+
+func TestEquivalentSourceFormsPreserveInputSemantics(t *testing.T) {
+	const yaml = "version: 1\nname: retained\nenabled: false\ncount: 0\nitems: []\noptional: null\n"
+	hostPath := filepath.Join(t.TempDir(), "main.yml")
+	if err := os.WriteFile(hostPath, []byte(yaml), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{"main.yml": &fstest.MapFile{Data: []byte(yaml)}})
+	context := &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &namespace}
+	for _, test := range []struct {
+		name    string
+		source  configkit.Source
+		context *configkit.FilesystemContext
+		kind    configkit.SourceKind
+	}{
+		{"host", configkit.File(hostPath), nil, configkit.SourceKindFile},
+		{"native", namespace.File("main.yml"), nil, configkit.SourceKindFilesystem},
+		{"bytes", configkit.Bytes("memory", []byte(yaml)), context, configkit.SourceKindBytes},
+		{"reader", configkit.Reader("stream", strings.NewReader(yaml)), context, configkit.SourceKindReader},
+		{"mapping", configkit.Mapping("programmatic", map[string]any{
+			"version": uint64(1), "name": "retained", "enabled": false, "count": 0,
+			"items": []any{}, "optional": nil,
+		}), context, configkit.SourceKindMapping},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			checks := 0
+			result, err := configkit.LoadLayered(configkit.Input{Source: test.source, Filesystem: test.context}, configkit.LayeredOptions[equivalentConfig]{
+				Definition: configkit.Definition[equivalentConfig]{Version: &configkit.VersionRule{
+					Path: configkit.Keys("version"), Check: func(version configkit.Version, validation *configkit.Validation) error {
+						checks++
+						if !version.Present || version.Text != "1" {
+							t.Errorf("version = %#v", version)
+						}
+						return nil
+					},
+				}},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			want := equivalentConfig{Version: 1, Name: "retained", Items: []string{}}
+			if !reflect.DeepEqual(result.Config, want) || checks != 1 {
+				t.Fatalf("config = %#v, checks %d", result.Config, checks)
+			}
+			for _, field := range []string{"version", "name", "enabled", "count", "items", "optional"} {
+				info, ok := result.Metadata().Lookup(configkit.Keys(field))
+				if !ok || !info.Explicit || info.Origin.Layer != configkit.LayerBase || info.Null != (field == "optional") {
+					t.Errorf("%s input info = %#v, %v", field, info, ok)
+				}
+			}
+			sources := result.Sources()
+			if len(sources) != 1 || sources[0].Kind != test.kind || sources[0].ID != "source-1" || sources[0].UnderlayIndex != 0 {
+				t.Fatalf("sources = %#v", sources)
+			}
+			resource, ok := result.SourceContext("source-1")
+			if !ok || resource.BaseDir != "." || resource.FS == nil {
+				t.Fatalf("source context = %#v, %v", resource, ok)
+			}
+		})
+	}
+}
+
+func TestRepeatedImportsAcrossTiersAndReaderConsumption(t *testing.T) {
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"left.yml":   &fstest.MapFile{Data: []byte("configkit: {imports: [{path: shared.yml}]}\n")},
+		"right.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: shared.yml}]}\n")},
+		"shared.yml": &fstest.MapFile{Data: []byte("name: retained\n")},
+	})
+	type config struct {
+		Name string `yaml:"name"`
+	}
+	left := configkit.Input{Source: namespace.File("left.yml"), Imports: true}
+	right := configkit.Input{Source: namespace.File("right.yml"), Imports: true}
+	_, err := configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{left, right}}},
+	})
+	if got := layeredErrorCode(t, err); got != configkit.CodeImportRepeated {
+		t.Fatalf("same-tier repeated import = %s", got)
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{left}}, {Inputs: []configkit.Input{right}}},
+	})
+	if err != nil || result.Config.Name != "retained" || len(result.Sources()) != 4 {
+		t.Fatalf("cross-tier repeated import = %#v, %v", result, err)
+	}
+	reader := configkit.Reader("single-use", strings.NewReader("name: retained\n"))
+	_, err = configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: reader}}}, {Inputs: []configkit.Input{{Source: reader}}}},
+	})
+	if got := layeredErrorCode(t, err); got != configkit.CodeSourceReaderUsed {
+		t.Fatalf("reader re-consumption = %s", got)
+	}
+}
+
+func TestReplacedSecretRemainsAbsentFromSafeOutput(t *testing.T) {
+	type config struct {
+		Secret string `yaml:"secret"`
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte("secret: new-secret-canary\n"))}, configkit.LayeredOptions[config]{
+		Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"secret": "old-secret-canary"})}}}},
+		Definition: configkit.Definition[config]{Secret: func(path configkit.Path) bool { return path.String() == "/secret" }},
+	})
+	if err != nil || result.Config.Secret != "new-secret-canary" {
+		t.Fatalf("secret load = %#v, %v", result, err)
+	}
+	var effective, history bytes.Buffer
+	if err := result.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	if err := result.WriteExplanation(&history, configkit.Keys("secret")); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(effective.String()+history.String(), "old-secret-canary") || strings.Contains(effective.String()+history.String(), "new-secret-canary") {
+		t.Fatalf("secret output leaked: %q %q", effective.String(), history.String())
+	}
+	_, err = configkit.LoadLayered(configkit.Input{Source: configkit.Bytes("main", []byte("secret: new-secret-canary\n"))}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"secret": "old-secret-canary"})}}}},
+		Definition: configkit.Definition[config]{Validate: func(config *config, validation *configkit.Validation) {
+			validation.Error(configkit.Keys("secret"), "invalid secret")
+		}},
+	})
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok {
+		t.Fatalf("validation error = %#v", err)
+	}
+	var diagnostics bytes.Buffer
+	if err := configkit.WriteDiagnostics(&diagnostics, loadError.Diagnostics()); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(diagnostics.String(), "secret-canary") {
+		t.Fatalf("diagnostic leaked secret: %q", diagnostics.String())
+	}
+}
+
+func layeredErrorCode(t *testing.T, err error) configkit.Code {
+	t.Helper()
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 {
+		t.Fatalf("load error = %#v", err)
+	}
+	return loadError.Diagnostics()[0].Code
+}

facade_filesystem_test.go

@@ -0,0 +1,142 @@
+package configkit_test
+
+import (
+	"embed"
+	"io/fs"
+	"reflect"
+	"strconv"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+//go:embed testdata/filesystem
+var filesystemFixtures embed.FS
+
+type filesystemFacadeConfig struct {
+	Items map[string]string `yaml:"items"`
+}
+
+func TestFacadeFilesystemSourceImportsOverlayAndReuse(t *testing.T) {
+	fixture, err := fs.Sub(filesystemFixtures, "testdata/filesystem")
+	if err != nil {
+		t.Fatal(err)
+	}
+	namespace := configkit.NewFilesystem("embedded", fixture)
+	copyOfNamespace := namespace
+	source := namespace.File("config/main.yml")
+	for range 2 {
+		result, err := configkit.Load(source, configkit.Options[filesystemFacadeConfig]{
+			Imports: true, Overlay: "testing", Filesystem: &configkit.FilesystemContext{FS: &copyOfNamespace},
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if want := (map[string]string{"main": "selected", "shared": "default"}); !reflect.DeepEqual(result.Config.Items, want) {
+			t.Fatalf("items = %#v, want %#v", result.Config.Items, want)
+		}
+		sources := result.Sources()
+		if len(sources) != 3 {
+			t.Fatalf("sources = %#v", sources)
+		}
+		for index, want := range []string{"config/main.yml", "shared.yml", "overlays/testing.yml"} {
+			if sources[index].ID != "source-"+strconv.Itoa(index+1) || sources[index].Path != want || sources[index].Kind != configkit.SourceKindFilesystem {
+				t.Errorf("source %d = %#v", index, sources[index])
+			}
+			if sources[index].Label != "embedded/"+want {
+				t.Errorf("source label = %q", sources[index].Label)
+			}
+		}
+		if sources[1].ImportedFrom == nil || sources[1].ImportedFrom.SourceID != sources[0].ID || sources[1].ImportedFrom.Line == 0 {
+			t.Fatalf("import location = %#v", sources[1].ImportedFrom)
+		}
+	}
+}
+
+func TestFacadeFilesystemContextsAndIdentity(t *testing.T) {
+	first := configkit.NewFilesystem("same", fstest.MapFS{
+		"root/config/main.yml":      &fstest.MapFile{Data: []byte("configkit: {imports: [{path: ../shared.yml}]}\n")},
+		"root/shared.yml":           &fstest.MapFile{Data: []byte("items: {value: first}\n")},
+		"root/overlays/testing.yml": &fstest.MapFile{Data: []byte("items: {overlay: first}\n")},
+	})
+	second := configkit.NewFilesystem("same", fstest.MapFS{
+		"root/config/main.yml": &fstest.MapFile{Data: []byte("items: {value: second}\n")},
+	})
+	context := &configkit.FilesystemContext{Root: "root", FS: &first}
+	result, err := configkit.Load(first.File("root/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Imports: true, Overlay: "testing", Filesystem: context})
+	if err != nil || result.Config.Items["value"] != "first" || result.Config.Items["overlay"] != "first" {
+		t.Fatalf("contained load = %#v, %v", result, err)
+	}
+	result, err = configkit.Load[filesystemFacadeConfig](second.File("root/config/main.yml"))
+	if err != nil || result.Config.Items["value"] != "second" {
+		t.Fatalf("same-label independent namespace = %#v, %v", result, err)
+	}
+	checkCode(t, second.File("root/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Filesystem: context}, configkit.CodeOptionsInvalid)
+	checkCode(t, first.File("root/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Filesystem: &configkit.FilesystemContext{Root: "root/config", FS: &first}, Imports: true}, configkit.CodeImportOutsideRoot)
+	checkCode(t, configkit.File("testdata/filesystem/config/main.yml"), configkit.Options[filesystemFacadeConfig]{Filesystem: context}, configkit.CodeOptionsInvalid)
+
+	for _, input := range []struct {
+		name   string
+		source configkit.Source
+		kind   configkit.SourceKind
+	}{
+		{"bytes", configkit.Bytes("memory", []byte("configkit: {imports: [{path: shared.yml}]}\n")), configkit.SourceKindBytes},
+		{"reader", configkit.Reader("stream", strings.NewReader("configkit: {imports: [{path: shared.yml}]}\n")), configkit.SourceKindReader},
+	} {
+		t.Run(input.name, func(t *testing.T) {
+			result, err := configkit.Load(input.source, configkit.Options[filesystemFacadeConfig]{Imports: true, Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: "root", FS: &first}})
+			if err != nil || result.Config.Items["value"] != "first" {
+				t.Fatalf("synthetic import = %#v, %v", result, err)
+			}
+			if got := result.Sources(); len(got) != 2 || got[0].Kind != input.kind || got[0].Path != "" || got[1].Kind != configkit.SourceKindFilesystem {
+				t.Fatalf("sources = %#v", got)
+			}
+		})
+	}
+	checkCode(t, configkit.Bytes("memory", []byte("items: {}\n")), configkit.Options[filesystemFacadeConfig]{Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: ".", FS: &first}}, configkit.CodeImportOutsideRoot)
+	result, err = configkit.Load(configkit.Bytes("memory", []byte("items: {value: main}\n")), configkit.Options[filesystemFacadeConfig]{
+		Overlay: "testing", Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: "root", FS: &first},
+	})
+	if err != nil || result.Config.Items["value"] != "main" || result.Config.Items["overlay"] != "first" {
+		t.Fatalf("synthetic selected overlay = %#v, %v", result, err)
+	}
+}
+
+func TestFacadeSourceKindsIncludeHostAndCLI(t *testing.T) {
+	result, err := configkit.Load(configkit.File("testdata/filesystem/shared.yml"), configkit.Options[filesystemFacadeConfig]{
+		Overrides: []configkit.Override{{Path: configkit.Keys("items", "shared"), Value: "changed", Flag: "--shared"}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	sources := result.Sources()
+	if len(sources) != 2 || sources[0].Kind != configkit.SourceKindFile || sources[0].Path == "" || sources[1].Kind != configkit.SourceKindCLI || sources[1].Path != "" {
+		t.Fatalf("host and CLI sources = %#v", sources)
+	}
+}
+
+func TestFacadeFilesystemConstructorDiagnostics(t *testing.T) {
+	var nilFilesystem fs.FS
+	var zero configkit.Filesystem
+	for _, source := range []configkit.Source{
+		zero.File("main.yml"),
+		configkit.NewFilesystem("", fstest.MapFS{}).File("main.yml"),
+		configkit.NewFilesystem("bad\nlabel", fstest.MapFS{}).File("main.yml"),
+		configkit.NewFilesystem("good", nilFilesystem).File("main.yml"),
+		configkit.NewFilesystem("good", fstest.MapFS{}).File("."),
+		configkit.NewFilesystem("good", fstest.MapFS{}).File("../outside.yml"),
+	} {
+		checkCode(t, source, configkit.Options[filesystemFacadeConfig]{}, configkit.CodeSourceInvalid)
+	}
+}
+
+func checkCode(t *testing.T, source configkit.Source, options configkit.Options[filesystemFacadeConfig], want configkit.Code) {
+	t.Helper()
+	_, err := configkit.Load(source, options)
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != want {
+		t.Fatalf("error = %#v, want %s", err, want)
+	}
+}

facade_inheritance_test.go

@@ -0,0 +1,284 @@
+package configkit_test
+
+import (
+	"bytes"
+	"io/fs"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type inheritedEntry struct {
+	Enabled  bool                         `yaml:"enabled"`
+	Count    int                          `yaml:"count"`
+	Resource string                       `yaml:"resource"`
+	Settings map[string]map[string]string `yaml:"settings"`
+}
+
+type inheritedConfiguration struct {
+	Version string                    `yaml:"version"`
+	Entries map[string]inheritedEntry `yaml:"entries"`
+}
+
+func TestFacadeInheritancePreservesPresenceOriginsAndHistory(t *testing.T) {
+	input := []byte(`entries:
+  parent:
+    enabled: false
+    count: 0
+    settings:
+      primary: {value: original}
+      secondary: {extends: primary}
+  child:
+    extends: parent
+    settings:
+      primary: {value: custom}
+`)
+	defaultRan := false
+	result, err := configkit.Load(configkit.Bytes("config.yml", input), configkit.Options[inheritedConfiguration]{Definition: configkit.Definition[inheritedConfiguration]{
+		Resolve: func(config *inheritedConfiguration, resolution *configkit.Resolution) error {
+			if !resolution.IsExplicit(configkit.Keys("entries", "child", "enabled")) {
+				t.Error("inherited false is not explicit")
+			}
+			info, found := resolution.InputInfo(configkit.Keys("entries", "child", "enabled"))
+			if !found || len(info.Origin.Inheritance) != 1 {
+				t.Errorf("resolver inherited origin = %#v, %v", info, found)
+			}
+			child := config.Entries["child"]
+			configkit.Default(resolution, configkit.Keys("entries", "child", "enabled"), &child.Enabled, true, "use fallback")
+			defaultRan = child.Enabled
+			config.Entries["child"] = child
+			return nil
+		},
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	child := result.Config.Entries["child"]
+	if defaultRan || child.Enabled || child.Count != 0 || child.Settings["secondary"]["value"] != "custom" {
+		t.Fatalf("inherited values = %#v, default ran %v", child, defaultRan)
+	}
+	path := configkit.Keys("entries", "child", "settings", "secondary", "value")
+	info, found := result.Metadata().Lookup(path)
+	if !found || !info.Explicit || len(info.Origin.Inheritance) != 1 ||
+		info.Origin.Inheritance[0].From.String() != "/entries/child/settings/primary/value" ||
+		info.Origin.Inheritance[0].To.String() != path.String() {
+		t.Fatalf("nested destination origin = %#v, %v", info, found)
+	}
+	info.Origin.Inheritance[0].Locations[0].SourceID = "mutated"
+	info, _ = result.Metadata().Lookup(path)
+	if info.Origin.Inheritance[0].Locations[0].SourceID == "mutated" {
+		t.Fatal("public inheritance record aliases inspection state")
+	}
+	if _, found := result.Metadata().Lookup(configkit.Keys("entries", "child", "extends")); found {
+		t.Fatal("consumed directive remained effective")
+	}
+	control, found := result.Explain(configkit.Keys("entries", "child", "extends"))
+	if !found || control.Effective.Present || len(control.History) < 2 ||
+		control.History[len(control.History)-1].Origin.Operation != configkit.ChangeInheritance {
+		t.Fatalf("consumed directive = %#v, %v", control, found)
+	}
+	explanation, found := result.Explain(path)
+	if !found || explanation.Effective.YAML != "custom\n" {
+		t.Fatalf("nested explanation = %#v, %v", explanation, found)
+	}
+	for _, change := range explanation.History {
+		if change.Origin.Operation == configkit.ChangeDecoding {
+			t.Fatalf("inherited value attributed to decoding: %#v", explanation.History)
+		}
+	}
+}
+
+func TestFacadeInheritanceLayeredPrecedenceAndSourceContext(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"config/main.yml":          &fstest.MapFile{Data: []byte("entries: {child: {extends: parent}}\n")},
+		"shared/parent.yml":        &fstest.MapFile{Data: []byte("entries: {parent: {resource: icon.txt, count: 1}}\n")},
+		"shared/flags.yml":         &fstest.MapFile{Data: []byte("entries: {parent: {enabled: false}}\n")},
+		"shared/icon.txt":          &fstest.MapFile{Data: []byte("parent-resource")},
+		"config/overlays/test.yml": &fstest.MapFile{Data: []byte("entries: {parent: {count: 2}}\n")},
+	})
+	result, err := configkit.LoadLayered(configkit.Input{Source: filesystem.File("config/main.yml")}, configkit.LayeredOptions[inheritedConfiguration]{
+		Underlays: []configkit.Underlay{
+			{Inputs: []configkit.Input{{Source: filesystem.File("shared/parent.yml")}}},
+			{Inputs: []configkit.Input{{Source: filesystem.File("shared/flags.yml")}}},
+		},
+		Overlay: "test", OverlayFilesystem: &configkit.FilesystemContext{Root: "config", BaseDir: "config", FS: &filesystem},
+		Overrides: []configkit.Override{{Path: configkit.Keys("entries", "child", "count"), Value: 3, Flag: "--count"}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	child := result.Config.Entries["child"]
+	if child.Count != 3 || child.Resource != "icon.txt" {
+		t.Fatalf("layered inheritance = %#v", child)
+	}
+	info, found := result.Metadata().Lookup(configkit.Keys("entries", "child", "resource"))
+	if !found || len(info.Origin.Inheritance) != 1 || len(info.Origin.Locations) != 1 {
+		t.Fatalf("resource origin = %#v, %v", info, found)
+	}
+	context, found := result.SourceContext(info.Origin.Locations[0].SourceID)
+	if !found || context.BaseDir != "shared" || context.FS == nil {
+		t.Fatalf("source context = %#v, %v", context, found)
+	}
+	data, err := fs.ReadFile(context.FS, context.BaseDir+"/"+child.Resource)
+	if err != nil || string(data) != "parent-resource" {
+		t.Fatalf("source-relative resource = %q, %v", data, err)
+	}
+	count, found := result.Metadata().Lookup(configkit.Keys("entries", "child", "count"))
+	if !found || count.Origin.Layer != configkit.LayerCLI || len(count.Origin.Inheritance) != 0 {
+		t.Fatalf("CLI replacement origin = %#v, %v", count, found)
+	}
+	var rendered bytes.Buffer
+	if err := result.WriteEffective(&rendered); err != nil || strings.Contains(rendered.String(), "extends:") {
+		t.Fatalf("effective rendering = %q, %v", rendered.String(), err)
+	}
+}
+
+func TestFacadeInheritanceMappingMatchesBytes(t *testing.T) {
+	values := map[string]any{"entries": map[string]any{
+		"parent": map[string]any{"enabled": false, "count": 0},
+		"child":  map[string]any{"extends": "parent"},
+	}}
+	mapped, err := configkit.Load[inheritedConfiguration](configkit.Mapping("mapping", values))
+	if err != nil {
+		t.Fatal(err)
+	}
+	fromYAML, err := configkit.Load[inheritedConfiguration](configkit.Bytes("config.yml", []byte("entries: {parent: {enabled: false, count: 0}, child: {extends: parent}}\n")))
+	if err != nil || !reflect.DeepEqual(mapped.Config, fromYAML.Config) {
+		t.Fatalf("mapping and bytes differ: %#v, %#v, %v", mapped, fromYAML, err)
+	}
+}
+
+func TestFacadeInheritanceProtectsVersionOwnership(t *testing.T) {
+	for _, test := range []struct {
+		name, input string
+		path        configkit.Path
+		wantFailure bool
+	}{
+		{"inherited version", "entries: {parent: {count: 1}, child: {extends: parent}}\n", configkit.Keys("entries", "child", "count"), true},
+		{"overridden version", "entries: {parent: {count: 1}, child: {extends: parent, count: 1}}\n", configkit.Keys("entries", "child", "count"), true},
+		{"direct unchanged version", "entries: {parent: {enabled: false}, child: {extends: parent, count: 1}}\n", configkit.Keys("entries", "child", "count"), false},
+		{"consumed checked directive", "entries: {parent: {count: 1}, child: {extends: parent}}\n", configkit.Keys("entries", "child", "extends"), true},
+		{"independent top-level version", "version: v1\nentries: {parent: {count: 1}, child: {extends: parent}}\n", configkit.Keys("version"), false},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			checks := struct{ version, defaults, resolution, validation int }{}
+			result, err := configkit.Load(configkit.Bytes("version.yml", []byte(test.input)), configkit.Options[inheritedConfiguration]{Definition: configkit.Definition[inheritedConfiguration]{
+				Version: &configkit.VersionRule{Path: test.path, Check: func(configkit.Version, *configkit.Validation) error {
+					checks.version++
+					return nil
+				}},
+				Defaults: func() inheritedConfiguration { checks.defaults++; return inheritedConfiguration{} },
+				Resolve:  func(*inheritedConfiguration, *configkit.Resolution) error { checks.resolution++; return nil },
+				Validate: func(*inheritedConfiguration, *configkit.Validation) { checks.validation++ },
+			}})
+			if test.wantFailure {
+				if result != nil || err == nil {
+					t.Fatalf("protected version = %#v, %v", result, err)
+				}
+				loadError, ok := err.(*configkit.LoadError)
+				if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeInheritanceVersion ||
+					checks != (struct{ version, defaults, resolution, validation int }{version: 1}) {
+					t.Fatalf("version failure = %v, callbacks %#v", err, checks)
+				}
+			} else if err != nil || result == nil || checks != (struct{ version, defaults, resolution, validation int }{1, 1, 1, 1}) {
+				t.Fatalf("unchanged version = %#v, %v, callbacks %#v", result, err, checks)
+			}
+		})
+	}
+}
+
+func TestFacadeInheritanceFailureStopsLaterCallbacks(t *testing.T) {
+	checks := struct{ defaults, resolution, validation int }{}
+	result, err := configkit.Load(configkit.Bytes("invalid.yml", []byte("entries: {child: {extends: PRIVATE-REFERENCE-CANARY}}\n")), configkit.Options[inheritedConfiguration]{Definition: configkit.Definition[inheritedConfiguration]{
+		Defaults: func() inheritedConfiguration { checks.defaults++; return inheritedConfiguration{} },
+		Resolve:  func(*inheritedConfiguration, *configkit.Resolution) error { checks.resolution++; return nil },
+		Validate: func(*inheritedConfiguration, *configkit.Validation) { checks.validation++ },
+	}})
+	if result != nil || err == nil || checks != (struct{ defaults, resolution, validation int }{}) || strings.Contains(err.Error(), "PRIVATE-REFERENCE-CANARY") {
+		t.Fatalf("invalid inheritance = %#v, %v, callbacks %#v", result, err, checks)
+	}
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeInheritanceMissing {
+		t.Fatalf("missing reference diagnostics = %v", err)
+	}
+}
+
+func TestFacadeInheritanceResolvesAfterImportsAndCLIReferenceChange(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":   &fstest.MapFile{Data: []byte("configkit: {imports: [{path: parent.yml}]}\nentries: {child: {extends: first}}\n")},
+		"parent.yml": &fstest.MapFile{Data: []byte("entries: {first: {count: 1}, second: {count: 2}}\n")},
+	})
+	result, err := configkit.Load(filesystem.File("main.yml"), configkit.Options[inheritedConfiguration]{
+		Imports: true, Overrides: []configkit.Override{{Path: configkit.Keys("entries", "child", "extends"), Value: "second", Flag: "--extends"}},
+	})
+	if err != nil || result.Config.Entries["child"].Count != 2 {
+		t.Fatalf("import and CLI reference = %#v, %v", result, err)
+	}
+	history := result.Metadata().History(configkit.Keys("entries", "child", "extends"))
+	if len(history) < 3 || history[len(history)-1].Origin.Operation != configkit.ChangeInheritance {
+		t.Fatalf("superseded reference history = %#v", history)
+	}
+	conflicting := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":   &fstest.MapFile{Data: []byte("configkit: {imports: [{path: parent.yml}]}\nentries: {child: {extends: first}, first: {count: 3}}\n")},
+		"parent.yml": &fstest.MapFile{Data: []byte("entries: {first: {count: 1}}\n")},
+	})
+	_, err = configkit.Load[inheritedConfiguration](conflicting.File("main.yml"), configkit.Options[inheritedConfiguration]{Imports: true})
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != configkit.CodeComposeConflict {
+		t.Fatalf("strict conflict before expansion = %v", err)
+	}
+}
+
+func TestFacadeInheritanceSelectedOverlayCancelsRelationship(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":          &fstest.MapFile{Data: []byte("entries: {parent: {count: 7}, child: {extends: parent, enabled: false}}\n")},
+		"overlays/test.yml": &fstest.MapFile{Data: []byte("entries: {child: {extends: null}}\n")},
+	})
+	result, err := configkit.Load(filesystem.File("main.yml"), configkit.Options[inheritedConfiguration]{
+		Overlay: "test", Filesystem: &configkit.FilesystemContext{Root: ".", FS: &filesystem},
+	})
+	if err != nil || result.Config.Entries["child"].Count != 0 || result.Config.Entries["child"].Enabled {
+		if loadError, ok := err.(*configkit.LoadError); ok {
+			t.Fatalf("cancelled relationship = %#v, %v, diagnostics %#v", result, err, loadError.Diagnostics())
+		}
+		t.Fatalf("cancelled relationship = %#v, %v", result, err)
+	}
+	history := result.Metadata().History(configkit.Keys("entries", "child", "extends"))
+	if len(history) < 3 || history[len(history)-1].Origin.Operation != configkit.ChangeInheritance {
+		t.Fatalf("cancellation history = %#v", history)
+	}
+}
+
+func TestFacadeInheritedSecretsStayRedactedAfterLookup(t *testing.T) {
+	const canary = "INHERITED-RELEASE-TEST-CREDENTIAL"
+	t.Setenv("CONFIGKIT_INHERITED_TEST_SECRET", canary)
+	type entry struct {
+		Credential configkit.Secret `yaml:"credential"`
+		Plain      string           `yaml:"plain"`
+	}
+	type configuration struct {
+		Entries map[string]entry `yaml:"entries"`
+	}
+	result, err := configkit.Load[configuration](configkit.Bytes("secret.yml", []byte("entries: {parent: {credential: CONFIGKIT_INHERITED_TEST_SECRET, plain: visible}, child: {extends: parent}}\n")), configkit.Options[configuration]{Definition: configkit.Definition[configuration]{
+		Secret: func(path configkit.Path) bool { return path.String() == "/entries/parent/credential" },
+	}})
+	if err != nil || result.Config.Entries["child"].Credential.Value() != canary {
+		t.Fatalf("inherited credential = %#v, %v", result, err)
+	}
+	path := configkit.Keys("entries", "child", "credential")
+	explanation, found := result.Explain(path)
+	if !found || !explanation.Effective.Redacted || strings.Contains(explanation.Effective.YAML, canary) {
+		t.Fatalf("credential explanation = %#v, %v", explanation, found)
+	}
+	var rendered bytes.Buffer
+	if err := result.WriteEffective(&rendered); err != nil || strings.Contains(rendered.String(), canary) || !strings.Contains(rendered.String(), "visible") {
+		t.Fatalf("effective secret rendering = %q, %v", rendered.String(), err)
+	}
+	rendered.Reset()
+	if err := result.WriteExplanation(&rendered, path); err != nil || strings.Contains(rendered.String(), canary) {
+		t.Fatalf("historical secret rendering = %q, %v", rendered.String(), err)
+	}
+}

facade_layered_test.go

@@ -0,0 +1,243 @@
+package configkit_test
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strconv"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type layeredFacadeConfig struct {
+	Settings map[string]string `yaml:"settings"`
+	Count    int               `yaml:"count"`
+	Enabled  bool              `yaml:"enabled"`
+	List     []string          `yaml:"list"`
+	Optional *string           `yaml:"optional"`
+	Derived  string            `yaml:"derived"`
+}
+
+func TestFacadeLayeredMixedSourcesAndInspection(t *testing.T) {
+	mainPath := filepath.Join(t.TempDir(), "main.yml")
+	if err := os.WriteFile(mainPath, []byte("settings: {shared: main, secret: new-canary}\ncount: 0\nenabled: false\nlist: []\noptional: null\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	lower := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"main.yml":             &fstest.MapFile{Data: []byte("configkit: {imports: [{path: defaults.yml}]}\nsettings: {shared: lower, native: retained, secret: old-canary}\nlist: [lower]\n")},
+		"defaults.yml":         &fstest.MapFile{Data: []byte("settings: {imported: retained}\n")},
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("settings: {from_lower_namespace: retained}\n")},
+	})
+	selected := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("settings: {shared: overlay}\n")},
+	})
+	checks := struct{ defaults, resolution, validation int }{}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.File(mainPath)}, configkit.LayeredOptions[layeredFacadeConfig]{
+		Underlays: []configkit.Underlay{
+			{Inputs: []configkit.Input{
+				{Source: lower.File("main.yml"), Imports: true},
+				{Source: configkit.Mapping("programmatic-low", map[string]any{"settings": map[string]any{"mapping": "retained"}, "count": 7, "enabled": true})},
+				{Source: lower.File("overlays/testing.yml")},
+			}},
+			{Inputs: []configkit.Input{{Source: configkit.Mapping("programmatic-high", map[string]any{
+				"settings": map[string]any{"shared": "middle", "second": "retained"}, "list": []any{"middle"},
+			})}}},
+		},
+		Overlay: "testing", OverlayFilesystem: &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &selected},
+		Overrides: []configkit.Override{{Path: configkit.Keys("settings", "shared"), Value: "cli", Flag: "--shared"}},
+		Definition: configkit.Definition[layeredFacadeConfig]{
+			Defaults: func() layeredFacadeConfig { checks.defaults++; return layeredFacadeConfig{} },
+			Resolve: func(config *layeredFacadeConfig, resolution *configkit.Resolution) error {
+				checks.resolution++
+				if source, ok := resolution.Source("source-2"); !ok || source.Layer != configkit.LayerUnderlay || source.UnderlayIndex != 1 || source.Kind != configkit.SourceKindFilesystem {
+					t.Errorf("resolver source = %#v, %v", source, ok)
+				}
+				configkit.Default(resolution, configkit.Keys("derived"), &config.Derived, config.Settings["native"], "derive from native", configkit.Keys("settings", "native"))
+				return nil
+			},
+			Validate: func(config *layeredFacadeConfig, validation *configkit.Validation) { checks.validation++ },
+			Secret:   func(path configkit.Path) bool { return path.String() == "/settings/secret" },
+		},
+	})
+	if err != nil {
+		t.Fatalf("layered load: %v, diagnostics %#v", err, err.(*configkit.LoadError).Diagnostics())
+	}
+	wantSettings := map[string]string{
+		"shared": "cli", "secret": "new-canary", "native": "retained", "imported": "retained",
+		"mapping": "retained", "from_lower_namespace": "retained", "second": "retained",
+	}
+	if !reflect.DeepEqual(result.Config.Settings, wantSettings) || result.Config.Count != 0 || result.Config.Enabled || result.Config.List == nil || len(result.Config.List) != 0 || result.Config.Optional != nil || result.Config.Derived != "retained" {
+		t.Fatalf("configuration = %#v", result.Config)
+	}
+	if checks != (struct{ defaults, resolution, validation int }{1, 1, 1}) {
+		t.Fatalf("lifecycle calls = %#v", checks)
+	}
+	sources := result.Sources()
+	if len(sources) != 8 {
+		t.Fatalf("sources = %#v", sources)
+	}
+	wantLayers := []configkit.Layer{configkit.LayerBase, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerUnderlay, configkit.LayerOverlay, configkit.LayerCLI}
+	wantIndexes := []int{0, 1, 1, 1, 1, 2, 0, 0}
+	wantKinds := []configkit.SourceKind{configkit.SourceKindFile, configkit.SourceKindFilesystem, configkit.SourceKindFilesystem, configkit.SourceKindMapping, configkit.SourceKindFilesystem, configkit.SourceKindMapping, configkit.SourceKindFilesystem, configkit.SourceKindCLI}
+	for index, source := range sources {
+		if source.ID != "source-"+strconv.Itoa(index+1) || source.Layer != wantLayers[index] || source.UnderlayIndex != wantIndexes[index] || source.Kind != wantKinds[index] {
+			t.Errorf("source %d = %#v", index, source)
+		}
+	}
+	if sources[4].Path != sources[6].Path || sources[4].Label != sources[6].Label {
+		t.Fatalf("namespace-collision fixture changed: %#v, %#v", sources[4], sources[6])
+	}
+	sources[1].Label = "changed"
+	if result.Sources()[1].Label != "bundle/main.yml" {
+		t.Fatal("source records alias retained metadata")
+	}
+	for _, test := range []struct {
+		path string
+		want configkit.Layer
+	}{
+		{"/settings/native", configkit.LayerUnderlay},
+		{"/settings/shared", configkit.LayerCLI},
+		{"/derived", configkit.LayerResolution},
+	} {
+		path, err := configkit.ParsePath(test.path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		info, ok := result.Metadata().Lookup(path)
+		if !ok || info.Origin.Layer != test.want {
+			t.Errorf("%s metadata = %#v, %v", test.path, info, ok)
+		}
+	}
+	container, ok := result.Metadata().Lookup(configkit.Keys("settings"))
+	if !ok || len(container.Origin.Locations) < 4 {
+		t.Fatalf("composite mapping origin = %#v, %v", container, ok)
+	}
+	explanation, ok := result.Explain(configkit.Keys("settings", "shared"))
+	if !ok || len(explanation.History) < 5 || explanation.Origin.Layer != configkit.LayerCLI {
+		t.Fatalf("shared history = %#v, %v", explanation, ok)
+	}
+	var effective, rendered bytes.Buffer
+	if err := result.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	if err := result.WriteExplanation(&rendered, configkit.Keys("settings", "secret")); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(effective.String()+rendered.String(), "old-canary") || strings.Contains(effective.String()+rendered.String(), "new-canary") || !strings.Contains(rendered.String(), "redacted") {
+		t.Fatalf("secret rendering leaked: effective %q, explanation %q", effective.String(), rendered.String())
+	}
+}
+
+func TestFacadeLayeredDisplacedDescendantRemainsInspectable(t *testing.T) {
+	type config struct {
+		Choice string `yaml:"choice"`
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.Mapping("main", map[string]any{"choice": "flat"})}, configkit.LayeredOptions[config]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{
+			"choice": map[string]any{"nested": "historical"},
+		})}}}},
+	})
+	if err != nil || result.Config.Choice != "flat" {
+		t.Fatalf("replacement load = %#v, %v", result, err)
+	}
+	path := configkit.Keys("choice", "nested")
+	explanation, ok := result.Explain(path)
+	if !ok || explanation.Effective.Present || len(explanation.History) == 0 || explanation.History[0].Origin.Layer != configkit.LayerUnderlay {
+		t.Fatalf("deleted descendant = %#v, %v", explanation, ok)
+	}
+}
+
+func TestFacadeLayeredZeroUnderlaysMatchesOrdinaryLoad(t *testing.T) {
+	filesystem := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("value: selected\n")},
+	})
+	context := &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &filesystem}
+	source := configkit.Bytes("main", []byte("value: base\n"))
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	ordinary, err := configkit.Load(source, configkit.Options[config]{Overlay: "testing", Filesystem: context})
+	if err != nil {
+		t.Fatal(err)
+	}
+	layered, err := configkit.LoadLayered(configkit.Input{Source: source}, configkit.LayeredOptions[config]{Overlay: "testing", OverlayFilesystem: context})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if ordinary.Config != layered.Config || !reflect.DeepEqual(ordinary.Sources(), layered.Sources()) {
+		t.Fatalf("ordinary = %#v, %#v; layered = %#v, %#v", ordinary.Config, ordinary.Sources(), layered.Config, layered.Sources())
+	}
+	var first, second bytes.Buffer
+	if err := ordinary.WriteEffective(&first); err != nil {
+		t.Fatal(err)
+	}
+	if err := layered.WriteEffective(&second); err != nil {
+		t.Fatal(err)
+	}
+	if first.String() != second.String() {
+		t.Fatalf("effective ordinary %q, layered %q", first.String(), second.String())
+	}
+	if ordinaryExplanation, ok := ordinary.Explain(configkit.Keys("value")); !ok {
+		t.Fatal("ordinary explanation missing")
+	} else if layeredExplanation, ok := layered.Explain(configkit.Keys("value")); !ok || !reflect.DeepEqual(ordinaryExplanation, layeredExplanation) {
+		t.Fatalf("explanations differ: %#v, %#v", ordinaryExplanation, layeredExplanation)
+	}
+}
+
+func TestFacadeLayeredOptionAndVersionFailures(t *testing.T) {
+	type config struct {
+		Version string `yaml:"version"`
+	}
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"overlays/testing.yml": &fstest.MapFile{Data: []byte("{}\n")},
+	})
+	for _, test := range []struct {
+		name    string
+		main    configkit.Input
+		options []configkit.LayeredOptions[config]
+		want    configkit.Code
+	}{
+		{"zero source", configkit.Input{}, nil, configkit.CodeSourceInvalid},
+		{"multiple options", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{}, {}}, configkit.CodeOptionsInvalid},
+		{"missing overlay context", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{Overlay: "testing"}}, configkit.CodeOptionsInvalid},
+		{"selected file already acquired", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{
+			Overlay: "testing", OverlayFilesystem: &configkit.FilesystemContext{Root: ".", BaseDir: ".", FS: &namespace},
+			Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{Source: namespace.File("overlays/testing.yml")}}}},
+		}}, configkit.CodeOverlayRepeated},
+		{"underlay version", configkit.Input{Source: configkit.NoSource()}, []configkit.LayeredOptions[config]{{
+			Definition: configkit.Definition[config]{Version: &configkit.VersionRule{Path: configkit.Keys("version"), Check: func(configkit.Version, *configkit.Validation) error { return nil }}},
+			Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"version": "forbidden"})}}}},
+		}}, configkit.CodeUnderlayVersion},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			_, err := configkit.LoadLayered(test.main, test.options...)
+			loadError, ok := err.(*configkit.LoadError)
+			if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != test.want {
+				t.Fatalf("error = %#v, want %s", err, test.want)
+			}
+		})
+	}
+}
+
+func TestFacadeLayeredExplicitHostOverlayWithoutMain(t *testing.T) {
+	root := t.TempDir()
+	if err := os.Mkdir(filepath.Join(root, "overlays"), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(root, "overlays", "testing.yml"), []byte("value: selected\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	type config struct {
+		Value string `yaml:"value"`
+	}
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[config]{
+		Overlay: "testing", OverlayFilesystem: &configkit.FilesystemContext{Root: root, BaseDir: root},
+	})
+	if err != nil || result.Config.Value != "selected" || len(result.Sources()) != 1 || result.Sources()[0].Kind != configkit.SourceKindFile {
+		t.Fatalf("explicit host overlay = %#v, %v", result, err)
+	}
+}

facade_mapping_test.go

@@ -0,0 +1,133 @@
+package configkit_test
+
+import (
+	"math"
+	"reflect"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type mappingFacadeConfig struct {
+	Enabled  bool     `yaml:"enabled"`
+	Count    uint64   `yaml:"count"`
+	Optional *string  `yaml:"optional"`
+	Items    []string `yaml:"items"`
+	Name     string   `yaml:"name"`
+}
+
+func TestFacadeMappingSnapshotsAndMatchesYAML(t *testing.T) {
+	items := []any{"one", "two"}
+	values := map[string]any{
+		"name": "original", "enabled": false, "count": ^uint64(0),
+		"optional": nil, "items": items,
+	}
+	source := configkit.Mapping("programmatic", values)
+	values["name"] = "mutated"
+	items[0] = "mutated"
+	for range 2 {
+		result, err := configkit.Load[mappingFacadeConfig](source)
+		if err != nil {
+			t.Fatal(err)
+		}
+		want := mappingFacadeConfig{Count: ^uint64(0), Items: []string{"one", "two"}, Name: "original"}
+		if !reflect.DeepEqual(result.Config, want) {
+			t.Fatalf("mapping config = %#v, want %#v", result.Config, want)
+		}
+		if sources := result.Sources(); len(sources) != 1 || sources[0].ID != "source-1" || sources[0].Kind != configkit.SourceKindMapping || sources[0].Layer != configkit.LayerBase || sources[0].Path != "" {
+			t.Fatalf("mapping sources = %#v", sources)
+		}
+		info, ok := result.Metadata().Lookup(configkit.Keys("name"))
+		if !ok || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != "source-1" || info.Origin.Locations[0].Line != 0 {
+			t.Fatalf("mapping name origin = %#v, %v", info, ok)
+		}
+		fromYAML, err := configkit.Load[mappingFacadeConfig](configkit.Bytes("equivalent", []byte("enabled: false\ncount: 18446744073709551615\noptional: null\nitems: [one, two]\nname: original\n")))
+		if err != nil || !reflect.DeepEqual(result.Config, fromYAML.Config) {
+			t.Fatalf("equivalent YAML = %#v, %v", fromYAML, err)
+		}
+	}
+}
+
+func TestFacadeMappingNullEmptyAndVersion(t *testing.T) {
+	result, err := configkit.Load[mappingFacadeConfig](configkit.Mapping("empty", map[string]any{}))
+	if err != nil || len(result.Sources()) != 1 {
+		t.Fatalf("empty mapping = %#v, %v", result, err)
+	}
+	var nilSequence []any
+	var nilMap map[string]any
+	var checked string
+	result, err = configkit.Load(configkit.Mapping("nullable", map[string]any{
+		"items": nilSequence, "optional": nilMap, "enabled": false, "count": uint64(0),
+	}), configkit.Options[mappingFacadeConfig]{Definition: configkit.Definition[mappingFacadeConfig]{
+		Version: &configkit.VersionRule{Path: configkit.Keys("count"), Check: func(version configkit.Version, validation *configkit.Validation) error {
+			checked = version.Text
+			return nil
+		}},
+	}})
+	if err != nil || checked != "0" || result.Config.Items != nil || result.Config.Optional != nil {
+		t.Fatalf("null and zero mapping = %#v, checked %q, %v", result, checked, err)
+	}
+	info, ok := result.Metadata().Lookup(configkit.Keys("items"))
+	if !ok || !info.Null || !info.Explicit {
+		t.Fatalf("null metadata = %#v, %v", info, ok)
+	}
+}
+
+func TestFacadeMappingImportsUseExplicitContext(t *testing.T) {
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"root/shared.yml": &fstest.MapFile{Data: []byte("name: imported\n")},
+	})
+	source := configkit.Mapping("programmatic", map[string]any{
+		"configkit": map[string]any{"imports": []any{map[string]any{"path": "shared.yml"}}},
+		"enabled":   true,
+	})
+	result, err := configkit.Load(source, configkit.Options[mappingFacadeConfig]{
+		Imports: true, Filesystem: &configkit.FilesystemContext{Root: "root", BaseDir: "root", FS: &namespace},
+	})
+	if err != nil || result.Config.Name != "imported" || !result.Config.Enabled {
+		t.Fatalf("mapping import = %#v, %v", result, err)
+	}
+	if sources := result.Sources(); len(sources) != 2 || sources[0].Kind != configkit.SourceKindMapping || sources[1].Kind != configkit.SourceKindFilesystem || sources[1].Layer != configkit.LayerBase {
+		t.Fatalf("mapping import sources = %#v", sources)
+	}
+	checkMappingCode(t, source, configkit.Options[mappingFacadeConfig]{Imports: true}, configkit.CodeOptionsInvalid)
+	checkMappingCode(t, source, configkit.Options[mappingFacadeConfig]{}, configkit.CodeImportDisabled)
+}
+
+func TestFacadeMappingDeferredValueDiagnostics(t *testing.T) {
+	cycle := map[string]any{}
+	cycle["secret"] = cycle
+	for _, test := range []struct {
+		name   string
+		source configkit.Source
+		want   configkit.Code
+		path   string
+	}{
+		{"nil root", configkit.Mapping("nil", nil), configkit.CodeSourceInvalid, ""},
+		{"bad label", configkit.Mapping("bad\nlabel", map[string]any{}), configkit.CodeSourceInvalid, ""},
+		{"unsupported", configkit.Mapping("bad", map[string]any{"secret": struct{ Value string }{"canary-value"}}), configkit.CodeSourceInvalid, "/secret"},
+		{"nonfinite", configkit.Mapping("bad", map[string]any{"count": math.NaN()}), configkit.CodeSourceInvalid, "/count"},
+		{"cycle", configkit.Mapping("bad", cycle), configkit.CodeSourceInvalid, "/secret"},
+		{"too many bytes", configkit.Mapping("bad", map[string]any{"name": strings.Repeat("x", 8<<20+1)}), configkit.CodeSourceLimitExceeded, "/name"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			loadError := checkMappingCode(t, test.source, configkit.Options[mappingFacadeConfig]{}, test.want)
+			diagnostic := loadError.Diagnostics()[0]
+			if diagnostic.Path.String() != test.path || diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || strings.Contains(diagnostic.Message, "canary-value") {
+				t.Fatalf("diagnostic = %#v", diagnostic)
+			}
+		})
+	}
+}
+
+func checkMappingCode(t *testing.T, source configkit.Source, options configkit.Options[mappingFacadeConfig], want configkit.Code) *configkit.LoadError {
+	t.Helper()
+	_, err := configkit.Load(source, options)
+	loadError, ok := err.(*configkit.LoadError)
+	if !ok || len(loadError.Diagnostics()) == 0 || loadError.Diagnostics()[0].Code != want {
+		t.Fatalf("error = %#v, want %s", err, want)
+	}
+	return loadError
+}

facade_resource_context_test.go

@@ -0,0 +1,221 @@
+package configkit_test
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+type observedResourceFS struct {
+	files  fstest.MapFS
+	opens  int
+	closed bool
+}
+
+func (f *observedResourceFS) Open(name string) (fs.File, error) {
+	f.opens++
+	if f.closed {
+		return nil, fs.ErrClosed
+	}
+	return f.files.Open(name)
+}
+
+type resourceFacadeConfig struct {
+	Native   string `yaml:"native"`
+	Shared   string `yaml:"shared"`
+	Host     string `yaml:"host"`
+	Selected string `yaml:"selected"`
+}
+
+func TestFacadeSourceContextsFollowOriginsWithoutQueryIO(t *testing.T) {
+	host := t.TempDir()
+	if err := os.WriteFile(filepath.Join(host, "main.yml"), []byte("host: host.txt\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(host, "host.txt"), []byte("host-v1"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	borrowed := &observedResourceFS{files: fstest.MapFS{
+		"bundle/config/defaults.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: ../shared.yml}]}\nnative: icon.txt\n")},
+		"bundle/shared.yml":           &fstest.MapFile{Data: []byte("shared: shared.txt\n")},
+		"bundle/overlays/test.yml":    &fstest.MapFile{Data: []byte("selected: overlay.txt\n")},
+		"bundle/config/icon.txt":      &fstest.MapFile{Data: []byte("icon-v1")},
+		"bundle/shared.txt":           &fstest.MapFile{Data: []byte("shared-v1")},
+		"bundle/overlays/overlay.txt": &fstest.MapFile{Data: []byte("overlay-v1")},
+	}}
+	namespace := configkit.NewFilesystem("bundle", borrowed)
+	var resolverContext configkit.ResourceContext
+	result, err := configkit.LoadLayered(configkit.Input{Source: configkit.File(filepath.Join(host, "main.yml"))}, configkit.LayeredOptions[resourceFacadeConfig]{
+		Underlays: []configkit.Underlay{{Inputs: []configkit.Input{{
+			Source: namespace.File("bundle/config/defaults.yml"), Imports: true,
+			Filesystem: &configkit.FilesystemContext{Root: "bundle", FS: &namespace},
+		}}}},
+		Overlay: "test", OverlayFilesystem: &configkit.FilesystemContext{Root: "bundle", BaseDir: "bundle", FS: &namespace},
+		Definition: configkit.Definition[resourceFacadeConfig]{Resolve: func(config *resourceFacadeConfig, resolution *configkit.Resolution) error {
+			before := borrowed.opens
+			var ok bool
+			resolverContext, ok = resolution.SourceContext("source-2")
+			if !ok || resolverContext.BaseDir != "config" || resolverContext.FS == nil {
+				t.Errorf("resolver context = %#v, %v", resolverContext, ok)
+			}
+			if _, ok := resolution.SourceContext("missing"); ok || borrowed.opens != before {
+				t.Error("resolution context query performed I/O or guessed an unknown source")
+			}
+			return nil
+		}},
+	})
+	if err != nil {
+		t.Fatalf("load: %v, diagnostics %#v", err, err.(*configkit.LoadError).Diagnostics())
+	}
+	queriesBefore := borrowed.opens
+	for _, test := range []struct {
+		field, wantBase, wantData string
+	}{
+		{"native", "config", "icon-v1"},
+		{"shared", ".", "shared-v1"},
+		{"selected", "overlays", "overlay-v1"},
+		{"host", ".", "host-v1"},
+	} {
+		origin, ok := result.Metadata().Lookup(configkit.Keys(test.field))
+		if !ok || len(origin.Origin.Locations) != 1 {
+			t.Fatalf("%s origin = %#v, %v", test.field, origin, ok)
+		}
+		context, ok := result.SourceContext(origin.Origin.Locations[0].SourceID)
+		if !ok || context.FS == nil || context.BaseDir != test.wantBase {
+			t.Fatalf("%s context = %#v, %v", test.field, context, ok)
+		}
+		if borrowed.opens != queriesBefore {
+			t.Fatal("result SourceContext query performed filesystem I/O")
+		}
+		resourceName := path.Join(context.BaseDir, map[string]string{
+			"native": result.Config.Native, "shared": result.Config.Shared,
+			"selected": result.Config.Selected, "host": result.Config.Host,
+		}[test.field])
+		data, err := fs.ReadFile(context.FS, resourceName)
+		if err != nil || string(data) != test.wantData {
+			t.Fatalf("%s resource = %q, %v", test.field, data, err)
+		}
+		queriesBefore = borrowed.opens
+	}
+	if _, ok := result.SourceContext(""); ok {
+		t.Fatal("empty source ID returned a context")
+	}
+	var nilResult *configkit.Result[resourceFacadeConfig]
+	if _, ok := nilResult.SourceContext("source-2"); ok {
+		t.Fatal("nil result returned a context")
+	}
+	if _, err := fs.ReadFile(resolverContext.FS, "../shared.txt"); !errors.Is(err, fs.ErrInvalid) {
+		t.Fatalf("native root escape = %v", err)
+	}
+	borrowed.files["bundle/config/icon.txt"].Data = []byte("icon-v2")
+	data, err := fs.ReadFile(resolverContext.FS, "config/icon.txt")
+	if err != nil || string(data) != "icon-v2" {
+		t.Fatalf("updated native resource = %q, %v", data, err)
+	}
+	borrowed.closed = true
+	if result.Config.Native != "icon.txt" {
+		t.Fatal("closing caller filesystem altered loaded configuration")
+	}
+	if _, err := fs.ReadFile(resolverContext.FS, "config/icon.txt"); !errors.Is(err, fs.ErrClosed) {
+		t.Fatalf("closed caller filesystem error = %v", err)
+	}
+}
+
+func TestFacadeHostContextSurvivesConfigurationDeletion(t *testing.T) {
+	root := t.TempDir()
+	main := filepath.Join(root, "main.yml")
+	resource := filepath.Join(root, "asset.txt")
+	if err := os.WriteFile(main, []byte("native: asset.txt\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(resource, []byte("first"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	result, err := configkit.Load[resourceFacadeConfig](configkit.File(main))
+	if err != nil {
+		t.Fatal(err)
+	}
+	context, ok := result.SourceContext("source-1")
+	if !ok || context.BaseDir != "." {
+		t.Fatalf("host context = %#v, %v", context, ok)
+	}
+	if err := os.Remove(main); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(resource, []byte("second"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	data, err := fs.ReadFile(context.FS, path.Join(context.BaseDir, result.Config.Native))
+	if err != nil || string(data) != "second" || result.Config.Native != "asset.txt" {
+		t.Fatalf("resource after source deletion = %q, %v", data, err)
+	}
+	if _, err := fs.ReadFile(context.FS, "../asset.txt"); !errors.Is(err, fs.ErrInvalid) {
+		t.Fatalf("invalid host path = %v", err)
+	}
+	if err := os.Remove(resource); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := fs.ReadFile(context.FS, "asset.txt"); !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("missing later resource = %v", err)
+	}
+	outside := filepath.Join(t.TempDir(), "outside.txt")
+	if err := os.WriteFile(outside, []byte("outside"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(outside, filepath.Join(root, "escape")); err == nil {
+		if _, err := fs.ReadFile(context.FS, "escape"); !errors.Is(err, fs.ErrPermission) {
+			t.Fatalf("host symlink escape = %v", err)
+		}
+	}
+}
+
+func TestFacadeSyntheticSourceContextsAndContextlessSources(t *testing.T) {
+	namespace := configkit.NewFilesystem("bundle", fstest.MapFS{
+		"bundle/config/icon.txt": &fstest.MapFile{Data: []byte("icon")},
+	})
+	context := &configkit.FilesystemContext{Root: "bundle", BaseDir: "bundle/config", FS: &namespace}
+	for _, test := range []struct {
+		name   string
+		source configkit.Source
+	}{
+		{"bytes", configkit.Bytes("memory", []byte("native: icon.txt\n"))},
+		{"reader", configkit.Reader("stream", strings.NewReader("native: icon.txt\n"))},
+		{"mapping", configkit.Mapping("values", map[string]any{"native": "icon.txt"})},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := configkit.Load(test.source, configkit.Options[resourceFacadeConfig]{Filesystem: context})
+			if err != nil {
+				t.Fatal(err)
+			}
+			resource, ok := result.SourceContext("source-1")
+			if !ok || resource.BaseDir != "config" {
+				t.Fatalf("synthetic context = %#v, %v", resource, ok)
+			}
+			data, err := fs.ReadFile(resource.FS, path.Join(resource.BaseDir, result.Config.Native))
+			if err != nil || string(data) != "icon" {
+				t.Fatalf("synthetic resource = %q, %v", data, err)
+			}
+		})
+	}
+	result, err := configkit.Load[resourceFacadeConfig](configkit.Bytes("plain", []byte("native: none\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := result.SourceContext("source-1"); ok {
+		t.Fatal("contextless byte source returned a context")
+	}
+	result, err = configkit.Load(configkit.NoSource(), configkit.Options[resourceFacadeConfig]{Overrides: []configkit.Override{{Path: configkit.Keys("native"), Value: "from-cli", Flag: "--native"}}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := result.SourceContext("source-1"); ok {
+		t.Fatal("CLI source returned a context")
+	}
+}

facade_underlay_origins_test.go

@@ -0,0 +1,107 @@
+package configkit_test
+
+import (
+	"reflect"
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestUnderlaySequenceDescendantOrigins(t *testing.T) {
+	for _, later := range []bool{false, true} {
+		name := "first tier"
+		if later {
+			name = "later tier"
+		}
+		t.Run(name, func(t *testing.T) {
+			bundle := configkit.NewFilesystem("bundle", fstest.MapFS{
+				"main.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: items.yml}]}\n")},
+				"items.yml": &fstest.MapFile{Data: []byte("items: [{name: first, nested: [false]}]\n")},
+			})
+			tiers := []configkit.Underlay{}
+			if later {
+				tiers = append(tiers, configkit.Underlay{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"other": true})}}})
+			}
+			tiers = append(tiers, configkit.Underlay{Inputs: []configkit.Input{{Source: bundle.File("main.yml"), Imports: true}}})
+			paths := []string{"/items", "/items/0", "/items/0/name", "/items/0/nested", "/items/0/nested/0"}
+			entry := map[string]configkit.Origin{}
+			result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.Mapping("main", map[string]any{"main": true})}, configkit.LayeredOptions[map[string]any]{
+				Underlays: tiers,
+				Definition: configkit.Definition[map[string]any]{Resolve: func(_ *map[string]any, resolution *configkit.Resolution) error {
+					for _, pointer := range paths {
+						path, _ := configkit.ParsePath(pointer)
+						info, ok := resolution.InputInfo(path)
+						if !ok {
+							t.Errorf("missing resolver input %s", pointer)
+						}
+						entry[pointer] = info.Origin
+					}
+					return nil
+				}},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			var sourceID string
+			for _, source := range result.Sources() {
+				if source.Path == "items.yml" {
+					sourceID = source.ID
+				}
+			}
+			if sourceID == "" {
+				t.Fatal("missing imported source")
+			}
+			for _, pointer := range paths {
+				path, _ := configkit.ParsePath(pointer)
+				info, ok := result.Metadata().Lookup(path)
+				if !ok || info.Origin.Layer != configkit.LayerUnderlay || len(info.Origin.Locations) != 1 || info.Origin.Locations[0].SourceID != sourceID {
+					t.Errorf("%s origin = %+v", pointer, info.Origin)
+				}
+				if !reflect.DeepEqual(entry[pointer], info.Origin) {
+					t.Errorf("%s resolver origin differs", pointer)
+				}
+				explanation, ok := result.Explain(path)
+				if !ok || !reflect.DeepEqual(explanation.Origin, info.Origin) {
+					t.Errorf("%s explanation origin differs", pointer)
+				}
+				history := result.Metadata().History(path)
+				if len(history) == 0 || !reflect.DeepEqual(history[len(history)-1].Origin, info.Origin) {
+					t.Errorf("%s history = %+v", pointer, history)
+				}
+			}
+		})
+	}
+}
+
+func TestUnderlayHistoryRetainsLaterMappingContributors(t *testing.T) {
+	result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.NoSource()}, configkit.LayeredOptions[map[string]any]{Underlays: []configkit.Underlay{
+		{Inputs: []configkit.Input{{Source: configkit.Mapping("one", map[string]any{"x": 1})}}},
+		{Inputs: []configkit.Input{{Source: configkit.Mapping("two", map[string]any{"x": map[string]any{}})}}},
+		{Inputs: []configkit.Input{{Source: configkit.Mapping("three", map[string]any{"x": map[string]any{"y": 2}})}}},
+	}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	path := configkit.Keys("x")
+	info, _ := result.Metadata().Lookup(path)
+	wantOrigin := configkit.Origin{Layer: configkit.LayerUnderlay, Locations: []configkit.Location{{SourceID: "source-2"}, {SourceID: "source-3"}}}
+	if !reflect.DeepEqual(info.Origin, wantOrigin) {
+		t.Fatalf("effective origin = %+v, want %+v", info.Origin, wantOrigin)
+	}
+	history := result.Metadata().History(path)
+	if len(history) != 3 || !reflect.DeepEqual(history[len(history)-1].Origin, info.Origin) {
+		t.Fatalf("history = %+v, effective origin = %+v", history, info.Origin)
+	}
+	explanation, ok := result.Explain(path)
+	if !ok || !reflect.DeepEqual(explanation.Origin, info.Origin) {
+		t.Fatal("explanation and metadata disagree")
+	}
+	if len(explanation.History) != 3 {
+		t.Fatalf("explanation history = %+v", explanation.History)
+	}
+	last := explanation.History[len(explanation.History)-1].Origin
+	if last.Layer != configkit.LayerUnderlay || len(last.Locations) != 1 || last.Locations[0].SourceID != "source-3" {
+		t.Fatalf("last explanation origin = %+v", last)
+	}
+}

go.mod

@@ -1,5 +1,13 @@
 module gitea.maximumdirect.net/eric/ConfigKit
 
-go 1.25.0
+go 1.26.0
 
 require go.yaml.in/yaml/v4 v4.0.0-rc.6
+
+require (
+	gitea.maximumdirect.net/eric/ReleaseKit v0.3.0 // indirect
+	github.com/yuin/goldmark/v2 v2.1.6 // indirect
+	golang.org/x/mod v0.40.0 // indirect
+)
+
+tool gitea.maximumdirect.net/eric/ReleaseKit/cmd/releasekit

go.sum

@@ -1,2 +1,8 @@
+gitea.maximumdirect.net/eric/ReleaseKit v0.3.0 h1:aTMn+50MEpHpHI2RoCCmJYsy7Ohrpa+RSva63Kq0/tU=
+gitea.maximumdirect.net/eric/ReleaseKit v0.3.0/go.mod h1:LVodqNQ3OIKv9NBq+fFeyMByd1lHY25YOmmpah/NaNI=
+github.com/yuin/goldmark/v2 v2.1.6 h1:igy0q9JcqovoFXFx/XjtU8sHVhcUHavPj2sA3EoKJUE=
+github.com/yuin/goldmark/v2 v2.1.6/go.mod h1:G6M4/qOFtfn01/o14BU1UR2Lo5N3S9Qo7xCuz/sHjGQ=
 go.yaml.in/yaml/v4 v4.0.0-rc.6 h1:1h7H1ohdUh93/FyE4YaDa1Zh64K6VVbjF4K6WUxMtH4=
 go.yaml.in/yaml/v4 v4.0.0-rc.6/go.mod h1:aZqd9kCMsGL7AuUv/m/PvWLdg5sjJsZ4oHDEnfPPfY0=
+golang.org/x/mod v0.40.0 h1:hUv+3cXcdRHz08UmSiOob7sadHig73uo5bkXxQ/tvUs=
+golang.org/x/mod v0.40.0/go.mod h1:0/weTWkPWGBikyTWAX3dkjVztMmBA5hM0DH6BElSupE=

inspection.go

@@ -39,7 +39,8 @@ type Explanation struct {
 }
 
 // Explain returns safe effective and chronological values for a known final or
-// historical path. Unknown and malformed paths return false.
+// historical path. Structural inheritance appears between source applications
+// and typed reconciliation. Unknown and malformed paths return false.
 func (r *Result[T]) Explain(path Path) (Explanation, bool) {
 	if r == nil {
 		return Explanation{}, false

internal/engine/assembly_bench_test.go

@@ -0,0 +1,119 @@
+package engine
+
+import (
+	"strconv"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func BenchmarkComposeWideMappings(b *testing.B) {
+	for _, width := range []int{128, 256, 512} {
+		b.Run(strconv.Itoa(width), func(b *testing.B) {
+			left := benchmarkCatalog("source-1", 0, width)
+			right := benchmarkCatalog("source-2", width, width)
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, diagnostics := composeDocuments(left, right); len(diagnostics) != 0 {
+					b.Fatal(diagnostics)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkOverlayRecursiveMappings(b *testing.B) {
+	for _, width := range []int{128, 256, 512} {
+		b.Run(strconv.Itoa(width), func(b *testing.B) {
+			base, diagnostics := composeDocuments(benchmarkCatalog("source-1", 0, width))
+			if len(diagnostics) != 0 {
+				b.Fatal(diagnostics)
+			}
+			overlay := benchmarkCatalog("source-2", width/2, width)
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, diagnostics := overlayDocuments(base, overlay); len(diagnostics) != 0 {
+					b.Fatal(diagnostics)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkOverlayManyApplications(b *testing.B) {
+	for _, count := range []int{32, 64, 128} {
+		inputs := make([]*document.Node, count)
+		for index := range inputs {
+			inputs[index] = benchmarkCatalog("source-2", index, 1)
+		}
+		benchmarkOverlayApplications(b, count, inputs)
+	}
+}
+
+func BenchmarkOverlayRepeatedReplacement(b *testing.B) {
+	for _, count := range []int{32, 64, 128} {
+		inputs := make([]*document.Node, count)
+		for index := range inputs {
+			location := document.Location{SourceID: "source-2"}
+			value := &document.Node{Kind: document.String, Text: "replacement", Location: location}
+			if index%2 == 0 {
+				value = &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+					Key: "enabled", Value: &document.Node{Kind: document.Boolean, Text: "false", Location: location},
+				}}}
+			}
+			inputs[index] = &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+				Key: "catalog", Value: &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+					Key: "item-0", Value: value,
+				}}},
+			}}}
+		}
+		benchmarkOverlayApplications(b, count, inputs)
+	}
+}
+
+func benchmarkOverlayApplications(b *testing.B, count int, inputs []*document.Node) {
+	base, diagnostics := composeDocuments(benchmarkCatalog("source-1", 0, 1))
+	if len(diagnostics) != 0 {
+		b.Fatal(diagnostics)
+	}
+	for _, mode := range []string{"owned", "copy-preserving"} {
+		b.Run(mode+"/"+strconv.Itoa(count), func(b *testing.B) {
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if mode == "owned" {
+					assembly := newAssemblyAccumulator(cloneComposedDocument(base))
+					for _, input := range inputs {
+						if diagnostics := assembly.applyOverlay(input, nil, nil, LayerOverlay); len(diagnostics) != 0 {
+							b.Fatal(diagnostics)
+						}
+					}
+					continue
+				}
+				current := base
+				for _, input := range inputs {
+					current, diagnostics = overlayDocuments(current, input)
+					if len(diagnostics) != 0 {
+						b.Fatal(diagnostics)
+					}
+				}
+			}
+		})
+	}
+}
+
+func benchmarkCatalog(source string, first, width int) *document.Node {
+	location := document.Location{SourceID: source}
+	entries := make([]document.Entry, width)
+	for index := range entries {
+		entries[index] = document.Entry{
+			Key:   "item-" + strconv.Itoa(first+index),
+			Value: &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{Key: "enabled", Value: &document.Node{Kind: document.Boolean, Text: "true", Location: location}}}},
+		}
+	}
+	return &document.Node{Kind: document.Mapping, Location: location, Mapping: []document.Entry{{
+		Key: "catalog", Value: &document.Node{Kind: document.Mapping, Location: location, Mapping: entries},
+	}}}
+}

internal/engine/compose.go

@@ -15,6 +15,17 @@ type composedDocument struct {
 	inputNodes   int
 }
 
+// assemblyAccumulator owns one mutable working document. Acquired trees held by
+// history applications are separate from its root and remain unchanged.
+type assemblyAccumulator struct {
+	current *composedDocument
+	index   mappingIndex
+}
+
+func newAssemblyAccumulator(owned *composedDocument) *assemblyAccumulator {
+	return &assemblyAccumulator{current: owned}
+}
+
 // cloneComposedDocument detaches mutable composition and provenance state;
 // history application trees are immutable and retain their existing sharing.
 func cloneComposedDocument(base *composedDocument) *composedDocument {
@@ -58,8 +69,14 @@ func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic
 		collected.append(input, Path{})
 	}
 
+	checked := make(map[string]struct{}, len(collected.byPath))
 	for _, claim := range collected.ordered {
-		claims := collected.byPath[claim.path.String()]
+		pointer := claim.path.String()
+		if _, seen := checked[pointer]; seen {
+			continue
+		}
+		checked[pointer] = struct{}{}
+		claims := collected.byPath[pointer]
 		if len(claims) < 2 || allMappingClaims(claims) {
 			continue
 		}
@@ -67,21 +84,43 @@ func composeDocuments(inputs ...*document.Node) (*composedDocument, []Diagnostic
 	}
 
 	root := document.Clone(inputs[0])
+	index := new(mappingIndex)
 	for _, input := range inputs[1:] {
-		mergeDisjointMappings(root, input)
+		mergeDisjointMappings(root, input, index)
 	}
 	applications := make([]historyApplication, 0, len(inputs))
 	for _, input := range inputs {
 		applications = append(applications, historyApplication{layer: LayerBase, value: input})
 	}
+	origins := compositionOrigins(collected.ordered)
+	completeCompositionOrigins(root, Path{}, origins)
 	return &composedDocument{
 		root:         root,
 		claims:       cloneCompositionClaims(collected.ordered),
-		origins:      compositionOrigins(collected.ordered),
+		origins:      origins,
 		applications: applications,
 	}, nil
 }
 
+// Conflict claims treat sequences atomically, but inspection needs origins for
+// every descendant. Preserve composite mapping claims while filling those gaps.
+func completeCompositionOrigins(node *document.Node, path Path, origins map[string]Origin) {
+	pointer := path.String()
+	if _, present := origins[pointer]; !present {
+		origins[pointer] = Origin{Layer: LayerBase, Locations: []Location{publicLocation(node.Location)}}
+	}
+	switch node.Kind {
+	case document.Mapping:
+		for _, entry := range node.Mapping {
+			completeCompositionOrigins(entry.Value, path.Key(entry.Key), origins)
+		}
+	case document.Sequence:
+		for index, child := range node.Sequence {
+			completeCompositionOrigins(child, path.Index(index), origins)
+		}
+	}
+}
+
 func (c *compositionClaims) append(node *document.Node, path Path) {
 	claim := compositionClaim{
 		path:     path.clone(),
@@ -109,18 +148,18 @@ func allMappingClaims(claims []compositionClaim) bool {
 	return true
 }
 
-func mergeDisjointMappings(target, incoming *document.Node) {
+func mergeDisjointMappings(target, incoming *document.Node, index *mappingIndex) {
 	for _, incomingEntry := range incoming.Mapping {
-		index := documentEntryIndex(target.Mapping, incomingEntry.Key)
-		if index < 0 {
+		position := index.find(target, incomingEntry.Key)
+		if position < 0 {
 			cloned := incomingEntry
 			cloned.Value = document.Clone(incomingEntry.Value)
-			target.Mapping = append(target.Mapping, cloned)
+			index.append(target, cloned)
 			continue
 		}
 		// Claim validation guarantees that the only remaining overlap is a
 		// mapping pair. Empty mappings naturally contribute no child entries.
-		mergeDisjointMappings(target.Mapping[index].Value, incomingEntry.Value)
+		mergeDisjointMappings(target.Mapping[position].Value, incomingEntry.Value, index)
 	}
 }
 

internal/engine/compose_test.go

@@ -116,6 +116,31 @@ func TestComposeDocumentsReportsAllClaimantsAtExactLocations(t *testing.T) {
 	assertCompositionConflict(t, diagnostics, "/group", []string{"source-1", "source-2", "source-3"})
 }
 
+func TestComposeDocumentsKeepsFirstConflictAndAppendedEntryOrder(t *testing.T) {
+	first := parseCompositionDocument(t, "source-1", "catalog:\n  first: 1\n  shared: one\n  later: one\n")
+	second := parseCompositionDocument(t, "source-2", "catalog:\n  second: 2\n  shared: two\n  later: two\n")
+	third := parseCompositionDocument(t, "source-3", "catalog:\n  third: 3\n  shared: three\n")
+	_, diagnostics := composeDocuments(first, second, third)
+	assertCompositionConflict(t, diagnostics, "/catalog/shared", []string{"source-1", "source-2", "source-3"})
+
+	composed, diagnostics := composeDocuments(
+		parseCompositionDocument(t, "source-1", "catalog:\n  first: 1\n"),
+		parseCompositionDocument(t, "source-2", "catalog:\n  second: 2\n"),
+		parseCompositionDocument(t, "source-3", "catalog:\n  third: 3\n"),
+	)
+	if len(diagnostics) != 0 {
+		t.Fatalf("composeDocuments() diagnostics = %#v", diagnostics)
+	}
+	entries := composed.root.Mapping[0].Value.Mapping
+	got := make([]string, len(entries))
+	for index, entry := range entries {
+		got[index] = entry.Key
+	}
+	if want := []string{"first", "second", "third"}; !reflect.DeepEqual(got, want) {
+		t.Fatalf("catalog keys = %#v, want %#v", got, want)
+	}
+}
+
 func TestComposeDocumentsTreatsEmptyMappingsAsChildlessContributions(t *testing.T) {
 	empty, diagnostics := composeDocuments(
 		parseCompositionDocument(t, "source-1", "{}\n"),

internal/engine/diagnostic.go

@@ -85,15 +85,28 @@ func canonicalDiagnostics(diagnostics []Diagnostic) []Diagnostic {
 	for i := range canonical {
 		slices.SortFunc(canonical[i].Related, compareLocation)
 	}
-	slices.SortFunc(canonical, compareDiagnostic)
+	type orderedDiagnostic struct {
+		diagnostic Diagnostic
+		pointer    string
+	}
+	ordered := make([]orderedDiagnostic, len(canonical))
+	for i, diagnostic := range canonical {
+		ordered[i] = orderedDiagnostic{diagnostic: diagnostic, pointer: diagnostic.Path.String()}
+	}
+	slices.SortFunc(ordered, func(left, right orderedDiagnostic) int {
+		return compareDiagnosticWithPaths(left.diagnostic, right.diagnostic, left.pointer, right.pointer)
+	})
+	for i, item := range ordered {
+		canonical[i] = item.diagnostic
+	}
 	return canonical
 }
 
-func compareDiagnostic(left, right Diagnostic) int {
+func compareDiagnosticWithPaths(left, right Diagnostic, leftPath, rightPath string) int {
 	if order := compareOptionalLocation(left.Primary, right.Primary); order != 0 {
 		return order
 	}
-	if order := cmp.Compare(left.Path.String(), right.Path.String()); order != 0 {
+	if order := cmp.Compare(leftPath, rightPath); order != 0 {
 		return order
 	}
 	if order := cmp.Compare(left.Code, right.Code); order != 0 {

internal/engine/diagnostic_test.go

@@ -90,6 +90,26 @@ func TestDiagnosticsAreCanonicalAndDetached(t *testing.T) {
 	}
 }
 
+func TestCanonicalDiagnosticsSortEscapedPaths(t *testing.T) {
+	primary := &Location{SourceID: "source-1", Line: 1, Column: 1}
+	diagnostics := []Diagnostic{
+		{Code: "same", Severity: SeverityError, Path: Keys("a/b"), Message: "same", Primary: primary},
+		{Code: "same", Severity: SeverityError, Path: Keys("a~b"), Message: "same", Primary: primary},
+		{Code: "z", Severity: SeverityError, Path: Keys("same"), Message: "same", Primary: primary},
+		{Code: "a", Severity: SeverityError, Path: Keys("same"), Message: "same", Primary: primary},
+	}
+	canonical := canonicalDiagnostics(diagnostics)
+	want := []string{"/a~0b", "/a~1b", "/same", "/same"}
+	for i, diagnostic := range canonical {
+		if got := diagnostic.Path.String(); got != want[i] {
+			t.Fatalf("diagnostic[%d] path = %q, want %q", i, got, want[i])
+		}
+	}
+	if canonical[2].Code != "a" || canonical[3].Code != "z" {
+		t.Fatalf("same-path code order = %q, %q", canonical[2].Code, canonical[3].Code)
+	}
+}
+
 func TestWriteDiagnosticsEscapesControls(t *testing.T) {
 	t.Parallel()
 

internal/engine/diff.go

@@ -0,0 +1,98 @@
+package engine
+
+import (
+	"bytes"
+	"errors"
+	"io"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+	yamladapter "gitea.maximumdirect.net/eric/ConfigKit/internal/yaml"
+)
+
+var errDiffInspectionUnavailable = errors.New("configkit: diff inspection unavailable")
+var errDiffSnapshotUnavailable = errors.New("configkit: diff snapshot unavailable")
+
+// Difference describes one canonical changed region with safe snapshots.
+type Difference struct {
+	Path          Path
+	Left          ValueSnapshot
+	Right         ValueSnapshot
+	HiddenChanged bool
+}
+
+type diffRegion struct {
+	path  Path
+	left  *document.Node
+	right *document.Node
+}
+
+func prepareDiffRegions(left, right *Inspection) ([]diffRegion, error) {
+	if left == nil || left.final == nil || right == nil || right.final == nil {
+		return nil, errDiffInspectionUnavailable
+	}
+
+	deltas := compareConfigurationValues(left.final, right.final)
+	regions := make([]diffRegion, 0, len(deltas))
+	for _, delta := range deltas {
+		leftNode, leftPresent := documentAtPathWithLookup(left.final, delta.path, left.immutableLookup)
+		if !leftPresent {
+			leftNode = nil
+		}
+		rightNode, rightPresent := documentAtPathWithLookup(right.final, delta.path, right.immutableLookup)
+		if !rightPresent {
+			rightNode = nil
+		}
+		regions = append(regions, diffRegion{path: delta.path, left: leftNode, right: rightNode})
+	}
+	return regions, nil
+}
+
+// Diff compares the final frozen values retained by two successful inspections.
+// Values are serialized only after applying the combined sensitivity of both loads.
+func Diff(left, right *Inspection) ([]Difference, error) {
+	regions, err := prepareDiffRegions(left, right)
+	if err != nil {
+		return nil, err
+	}
+	sensitivity := newDiffSensitivity(left.secrets, right.secrets)
+	differences := make([]Difference, 0, len(regions))
+	for _, region := range regions {
+		paired, supported := preparePairedDiffSnapshot(region.left, region.right, region.path, sensitivity)
+		if !supported {
+			return nil, errDiffSnapshotUnavailable
+		}
+		leftSnapshot, err := serializeDiffSnapshot(paired.left, paired.leftRedacted)
+		if err != nil {
+			return nil, errDiffSnapshotUnavailable
+		}
+		rightSnapshot, err := serializeDiffSnapshot(paired.right, paired.rightRedacted)
+		if err != nil {
+			return nil, errDiffSnapshotUnavailable
+		}
+		differences = append(differences, Difference{
+			Path:          region.path,
+			Left:          leftSnapshot,
+			Right:         rightSnapshot,
+			HiddenChanged: paired.hiddenChanged,
+		})
+	}
+	return differences, nil
+}
+
+func serializeDiffSnapshot(node *document.Node, redacted bool) (ValueSnapshot, error) {
+	if node == nil {
+		return ValueSnapshot{}, nil
+	}
+	var output bytes.Buffer
+	if err := writeDiffDocument(&output, node); err != nil {
+		return ValueSnapshot{}, errDiffSnapshotUnavailable
+	}
+	return ValueSnapshot{Present: true, Redacted: redacted, YAML: output.String()}, nil
+}
+
+func writeDiffDocument(output io.Writer, node *document.Node) error {
+	if err := yamladapter.Write(output, node); err != nil {
+		return errDiffSnapshotUnavailable
+	}
+	return nil
+}

internal/engine/diff_hidden_test.go

@@ -0,0 +1,130 @@
+package engine
+
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestPreparePairedDiffSnapshotReportsWhollyMaskedChanges(t *testing.T) {
+	left := &document.Node{Kind: document.String, Text: "private-before"}
+	right := &document.Node{Kind: document.String, Text: "private-after"}
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Keys("credential"), newDiffSensitivity(nil, map[string]bool{"/credential": true}),
+	)
+	if !supported || !paired.hiddenChanged || !paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("masked change = %#v, supported=%v", paired, supported)
+	}
+	assertDiffStringNode(t, paired.left, "[REDACTED]")
+	assertDiffStringNode(t, paired.right, "[REDACTED]")
+}
+
+func TestPreparePairedDiffSnapshotDetectsRawChangesBehindIdenticalMarkers(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "token: old-canary\n")
+	right := parseCompositionDocument(t, "right", "token: new-canary\n")
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "token"), mappingValue(t, right, "token"), Keys("token"),
+		newDiffSensitivity(map[string]bool{"/token": true}, nil),
+	)
+	if !supported || !paired.hiddenChanged || paired.left.Text != paired.right.Text {
+		t.Fatalf("identically rendered masked values lost raw change: %#v, supported=%v", paired, supported)
+	}
+}
+
+func TestPreparePairedDiffSnapshotCombinesPublicAndHiddenChanges(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "public: before\ntoken: old\n")
+	right := parseCompositionDocument(t, "right", "public: after\ntoken: new\n")
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Path{}, newDiffSensitivity(nil, map[string]bool{"/token": true}),
+	)
+	if !supported || !paired.hiddenChanged {
+		t.Fatalf("mixed public and hidden change = %#v, supported=%v", paired, supported)
+	}
+	assertDiffStringNode(t, mappingValue(t, paired.left, "public"), "before")
+	assertDiffStringNode(t, mappingValue(t, paired.right, "public"), "after")
+	assertDiffStringNode(t, mappingValue(t, paired.left, "token"), "[REDACTED]")
+	assertDiffStringNode(t, mappingValue(t, paired.right, "token"), "[REDACTED]")
+}
+
+func TestPreparePairedDiffSnapshotDoesNotFlagUnchangedMaskedValues(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "public: before\ntoken: unchanged\n")
+	right := parseCompositionDocument(t, "right", "public: after\ntoken: unchanged\n")
+	regions, err := prepareDiffRegions(diffTestInspection(left), diffTestInspection(right))
+	if err != nil || len(regions) != 1 || regions[0].path.String() != "/public" {
+		t.Fatalf("changed regions = %#v, err = %v; want only /public", regions, err)
+	}
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Path{}, newDiffSensitivity(map[string]bool{"/token": true}, nil),
+	)
+	if !supported || !paired.leftRedacted || !paired.rightRedacted || paired.hiddenChanged {
+		t.Fatalf("unchanged masked values affected hidden-change state: %#v, supported=%v", paired, supported)
+	}
+
+	leftSequence := parseCompositionDocument(t, "left-sequence", "items: [unchanged, before]\n")
+	rightSequence := parseCompositionDocument(t, "right-sequence", "items: [unchanged, after]\n")
+	sequence, supported := preparePairedDiffSnapshot(
+		mappingValue(t, leftSequence, "items"), mappingValue(t, rightSequence, "items"), Keys("items"),
+		newDiffSensitivity(nil, map[string]bool{"/items/0": true}),
+	)
+	if !supported || !sequence.leftRedacted || !sequence.rightRedacted || sequence.hiddenChanged {
+		t.Fatalf("unchanged masked sequence member affected hidden-change state: %#v, supported=%v", sequence, supported)
+	}
+}
+
+func TestPreparePairedDiffSnapshotReportsMaskedAdditionAndRemoval(t *testing.T) {
+	value := &document.Node{Kind: document.String, Text: "private"}
+	tests := []struct {
+		name        string
+		left, right *document.Node
+		leftHidden  bool
+		rightHidden bool
+	}{
+		{name: "addition", right: value, rightHidden: true},
+		{name: "removal", left: value, leftHidden: true},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			paired, supported := preparePairedDiffSnapshot(
+				test.left, test.right, Keys("token"), newDiffSensitivity(nil, map[string]bool{"/token": true}),
+			)
+			if !supported || !paired.hiddenChanged || paired.leftRedacted != test.leftHidden || paired.rightRedacted != test.rightHidden {
+				t.Fatalf("masked presence change = %#v, supported=%v", paired, supported)
+			}
+			if test.left == nil && paired.left != nil || test.right == nil && paired.right != nil {
+				t.Fatalf("absence became a value: %#v", paired)
+			}
+		})
+	}
+}
+
+func TestPreparePairedDiffSnapshotReportsHiddenSequenceLengthChange(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "items: [public]\n")
+	right := parseCompositionDocument(t, "right", "items: [public, private]\n")
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "items"), mappingValue(t, right, "items"), Keys("items"),
+		newDiffSensitivity(nil, map[string]bool{"/items/1": true}),
+	)
+	if !supported || !paired.hiddenChanged || paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("masked sequence length change = %#v, supported=%v", paired, supported)
+	}
+	if len(paired.left.Sequence) != 1 || len(paired.right.Sequence) != 2 {
+		t.Fatalf("safe sequence lengths = %d, %d; want 1, 2", len(paired.left.Sequence), len(paired.right.Sequence))
+	}
+	assertDiffStringNode(t, paired.right.Sequence[1], "[REDACTED]")
+}
+
+func TestPreparePairedDiffSnapshotClassificationAloneDoesNotCreateHiddenChange(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "token: same\n")
+	right := parseCompositionDocument(t, "right", "token: same\n")
+	regions, err := prepareDiffRegions(diffTestInspection(left), diffTestInspection(right))
+	if err != nil || len(regions) != 0 {
+		t.Fatalf("classification-independent changed regions = %#v, err = %v", regions, err)
+	}
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "token"), mappingValue(t, right, "token"), Keys("token"),
+		newDiffSensitivity(map[string]bool{"/token": true}, nil),
+	)
+	if !supported || !paired.leftRedacted || !paired.rightRedacted || paired.hiddenChanged {
+		t.Fatalf("classification alone reported hidden change: %#v, supported=%v", paired, supported)
+	}
+}

internal/engine/diff_operation_test.go

@@ -0,0 +1,282 @@
+package engine
+
+import (
+	"bytes"
+	"errors"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type diffIntegrationConfig struct {
+	Defaulted  string         `yaml:"defaulted"`
+	Dynamic    map[string]any `yaml:"dynamic"`
+	Credential Secret         `yaml:"credential" configkit:"secret"`
+	Derived    string         `yaml:"derived"`
+}
+
+type diffOptionalSecretConfig struct {
+	Optional *Secret `yaml:"optional" configkit:"secret"`
+}
+
+func TestDiffUsesFinalValuesAndDependencyDerivedSensitivity(t *testing.T) {
+	var resolveCalls int
+	options := Options[diffIntegrationConfig]{Definition: Definition[diffIntegrationConfig]{
+		Defaults: func() diffIntegrationConfig {
+			return diffIntegrationConfig{Defaulted: "from-default", Dynamic: map[string]any{"defaulted": "retained"}}
+		},
+		Resolve: func(configuration *diffIntegrationConfig, resolution *Resolution) error {
+			resolveCalls++
+			configuration.Derived = configuration.Dynamic["visible"].(string)
+			resolution.Record(Keys("derived"), ChangeNormalization, "derive from effective values", Keys("credential"))
+			return nil
+		},
+	}}
+	input := func(value string) Source {
+		return Bytes("effective.yml", []byte("dynamic: {visible: "+value+"}\ncredential: DIFF_ENGINE_SECRET\n"))
+	}
+	t.Setenv("DIFF_ENGINE_SECRET", "credential-left-canary")
+	left, err := testLoad(input("before"), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("DIFF_ENGINE_SECRET", "credential-right-canary")
+	right, err := testLoad(input("after"), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if left.Config.Defaulted != "from-default" || right.Config.Defaulted != "from-default" {
+		t.Fatalf("defaulted values = %q, %q", left.Config.Defaulted, right.Config.Defaulted)
+	}
+	if !left.Inspection.secrets["/credential"] || !left.Inspection.secrets["/derived"] ||
+		!right.Inspection.secrets["/credential"] || !right.Inspection.secrets["/derived"] {
+		t.Fatalf("typed or dependency-derived sensitivity was not retained: left=%#v right=%#v", left.Inspection.secrets, right.Inspection.secrets)
+	}
+	if node, present := documentAtPathWithLookup(left.Inspection.final, Keys("derived"), left.Inspection.immutableLookup); !present || node.Text != "before" {
+		t.Fatalf("left frozen derived value = %#v, present=%v", node, present)
+	}
+
+	differences, err := Diff(left.Inspection, right.Inspection)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(differences) != 2 || differences[0].Path.String() != "/derived" || differences[1].Path.String() != "/dynamic/visible" {
+		t.Fatalf("differences = %#v", differences)
+	}
+	if !differences[0].Left.Redacted || !differences[0].Right.Redacted || !differences[0].HiddenChanged ||
+		differences[0].Left.YAML != "'[REDACTED]'\n" || differences[0].Right.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("dependency-derived difference = %#v", differences[0])
+	}
+	if differences[1].Left.Redacted || differences[1].Right.Redacted || differences[1].HiddenChanged ||
+		differences[1].Left.YAML != "before\n" || differences[1].Right.YAML != "after\n" {
+		t.Fatalf("public dynamic difference = %#v", differences[1])
+	}
+	for _, value := range differences {
+		if strings.Contains(value.Left.YAML+value.Right.YAML, "credential-") {
+			t.Fatal("diff snapshot exposed a resolved credential")
+		}
+	}
+
+	var effective bytes.Buffer
+	if err := left.Inspection.WriteEffective(&effective); err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(effective.String(), "credential-left-canary") {
+		t.Fatal("standalone effective rendering exposed a credential after diff")
+	}
+	explanation, ok := left.Inspection.Explain(Keys("derived"))
+	if !ok || !explanation.Effective.Redacted || explanation.Effective.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("explanation changed after diff: %#v, ok=%v", explanation, ok)
+	}
+	if _, ok := left.Inspection.Metadata().Lookup(Keys("dynamic", "visible")); !ok {
+		t.Fatal("metadata lookup failed after diff")
+	}
+	if resolveCalls != 2 {
+		t.Fatalf("resolution callbacks ran during diff: %d calls", resolveCalls)
+	}
+
+	// Returned records can be modified without affecting a later comparison.
+	differences[0].Path.segments[0].key = "changed-by-caller"
+	differences[0].Left.YAML = "changed-by-caller"
+	t.Setenv("DIFF_ENGINE_SECRET", "credential-after-load-canary")
+	repeated, err := Diff(left.Inspection, right.Inspection)
+	if err != nil || len(repeated) != 2 || repeated[0].Path.String() != "/derived" || repeated[0].Left.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("repeated diff = %#v, err = %v", repeated, err)
+	}
+	if resolveCalls != 2 {
+		t.Fatalf("repeated diff reran resolution: %d calls", resolveCalls)
+	}
+
+	independent, err := testLoad(input("before"), options)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resolveCalls != 3 {
+		t.Fatalf("independent load did not retain normal resolution behavior: %d calls", resolveCalls)
+	}
+	if _, err := Diff(left.Inspection, independent.Inspection); err != nil {
+		t.Fatalf("independent load comparison = %v", err)
+	}
+}
+
+func TestDiffUsesReferenceNamesAndFrozenSecretState(t *testing.T) {
+	loadReference := func(name string) *testResult[diffOptionalSecretConfig] {
+		t.Helper()
+		result, err := testLoad[diffOptionalSecretConfig](Bytes("secret-reference.yml", []byte("optional: "+name+"\n")))
+		if err != nil {
+			t.Fatal(err)
+		}
+		return result
+	}
+	t.Setenv("DIFF_ROTATING_SECRET", "first-credential-canary")
+	left := loadReference("DIFF_ROTATING_SECRET")
+	t.Setenv("DIFF_ROTATING_SECRET", "second-credential-canary")
+	right := loadReference("DIFF_ROTATING_SECRET")
+	t.Setenv("DIFF_ROTATING_SECRET", "changed-after-load-canary")
+	differences, err := Diff(left.Inspection, right.Inspection)
+	if err != nil || len(differences) != 0 {
+		t.Fatalf("credential rotation behind one reference = %#v, err = %v", differences, err)
+	}
+	if node, present := documentAtPathWithLookup(left.Inspection.final, Keys("optional"), left.Inspection.immutableLookup); !present || node.Text != "DIFF_ROTATING_SECRET" {
+		t.Fatalf("frozen reference = %#v, present=%v", node, present)
+	}
+	if node, present := documentAtPathWithLookup(right.Inspection.final, Keys("optional"), right.Inspection.immutableLookup); !present || node.Text != "DIFF_ROTATING_SECRET" {
+		t.Fatalf("right frozen reference = %#v, present=%v", node, present)
+	}
+
+	t.Setenv("DIFF_OTHER_SECRET", "other-credential-canary")
+	other := loadReference("DIFF_OTHER_SECRET")
+	differences, err = Diff(left.Inspection, other.Inspection)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/optional" {
+		t.Fatalf("reference-name change = %#v, err = %v", differences, err)
+	}
+	if !differences[0].Left.Present || !differences[0].Right.Present || !differences[0].Left.Redacted ||
+		!differences[0].Right.Redacted || !differences[0].HiddenChanged ||
+		differences[0].Left.YAML != "'[REDACTED]'\n" || differences[0].Right.YAML != "'[REDACTED]'\n" {
+		t.Fatalf("classified reference-name diff = %#v", differences[0])
+	}
+
+	absent, err := testLoad[diffOptionalSecretConfig](Bytes("absent-secret.yml", []byte("{}\n")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if absent.Config.Optional != nil {
+		t.Fatalf("missing optional reference decoded as %#v", absent.Config.Optional)
+	}
+	if node, present := documentAtPathWithLookup(absent.Inspection.final, Keys("optional"), absent.Inspection.immutableLookup); !present || node.Kind != document.Null {
+		t.Fatalf("optional nil projection = %#v, present=%v", node, present)
+	}
+	differences, err = Diff(absent.Inspection, left.Inspection)
+	if err != nil || len(differences) != 1 || differences[0].Path.String() != "/optional" {
+		t.Fatalf("optional absence diff = %#v, err = %v", differences, err)
+	}
+	if !differences[0].Left.Present || !differences[0].Left.Redacted ||
+		!differences[0].Right.Present || !differences[0].Right.Redacted || !differences[0].HiddenChanged {
+		t.Fatalf("optional reference change snapshots = %#v", differences[0])
+	}
+	for _, value := range differences {
+		if strings.Contains(value.Left.YAML+value.Right.YAML, "credential-canary") {
+			t.Fatal("reference diff exposed a resolved credential")
+		}
+	}
+}
+
+func TestDiffProtectsHistoricalSensitiveRemovalAndReplacement(t *testing.T) {
+	type configuration struct {
+		Dynamic map[string]any `yaml:"dynamic"`
+	}
+	secretPath := Keys("dynamic", "token")
+	secretSelector := func(path Path) bool { return path.String() == secretPath.String() }
+	load := func(input string, resolve func(*configuration, *Resolution) error) *testResult[configuration] {
+		t.Helper()
+		result, err := testLoad(Bytes("historical-secret.yml", []byte(input)), Options[configuration]{Definition: Definition[configuration]{
+			Secret:  secretSelector,
+			Resolve: resolve,
+		}})
+		if err != nil {
+			t.Fatal(err)
+		}
+		return result
+	}
+
+	t.Run("removed historical value", func(t *testing.T) {
+		left := load("dynamic: {token: removed-canary}\n", func(configuration *configuration, resolution *Resolution) error {
+			delete(configuration.Dynamic, "token")
+			resolution.Record(secretPath, ChangeNormalization, "remove token")
+			return nil
+		})
+		right := load("dynamic: {token: current-canary}\n", nil)
+		if !left.Inspection.secrets[secretPath.String()] {
+			t.Fatal("removed historical selector was not retained")
+		}
+		if _, present := documentAtPathWithLookup(left.Inspection.final, secretPath, left.Inspection.immutableLookup); present {
+			t.Fatal("removed token remains in the effective left tree")
+		}
+		differences, err := Diff(left.Inspection, right.Inspection)
+		if err != nil || len(differences) != 1 || differences[0].Path.String() != secretPath.String() {
+			t.Fatalf("historical removal diff = %#v, err = %v", differences, err)
+		}
+		if differences[0].Left.Present || !differences[0].Right.Present || !differences[0].Right.Redacted || !differences[0].HiddenChanged {
+			t.Fatalf("historical removal snapshots = %#v", differences[0])
+		}
+		if strings.Contains(differences[0].Left.YAML+differences[0].Right.YAML, "canary") {
+			t.Fatal("historical removal diff exposed a sensitive value")
+		}
+	})
+
+	t.Run("replaced historical value", func(t *testing.T) {
+		left := load("dynamic: {token: original-canary}\n", func(configuration *configuration, resolution *Resolution) error {
+			configuration.Dynamic["token"] = "replacement-canary"
+			resolution.Record(secretPath, ChangeNormalization, "replace token")
+			return nil
+		})
+		right := load("dynamic: {token: other-canary}\n", nil)
+		if !left.Inspection.secrets[secretPath.String()] {
+			t.Fatal("replaced historical selector was not retained")
+		}
+		differences, err := Diff(left.Inspection, right.Inspection)
+		if err != nil || len(differences) != 1 || differences[0].Path.String() != secretPath.String() {
+			t.Fatalf("historical replacement diff = %#v, err = %v", differences, err)
+		}
+		if !differences[0].Left.Present || !differences[0].Right.Present || !differences[0].Left.Redacted ||
+			!differences[0].Right.Redacted || !differences[0].HiddenChanged {
+			t.Fatalf("historical replacement snapshots = %#v", differences[0])
+		}
+		if strings.Contains(differences[0].Left.YAML+differences[0].Right.YAML, "canary") {
+			t.Fatal("historical replacement diff exposed a sensitive value")
+		}
+	})
+}
+
+func TestDiffReturnsSafeErrorsWithoutRecords(t *testing.T) {
+	valid := diffTestInspection(&document.Node{Kind: document.String, Text: "safe"})
+	for _, test := range []struct {
+		name        string
+		left, right *Inspection
+	}{
+		{name: "nil left", right: valid},
+		{name: "nil right", left: valid},
+		{name: "missing final tree", left: &Inspection{}, right: valid},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			differences, err := Diff(test.left, test.right)
+			if differences != nil || err == nil || err != errDiffInspectionUnavailable || strings.Contains(err.Error(), "safe") {
+				t.Fatalf("differences, err = %#v, %v", differences, err)
+			}
+		})
+	}
+}
+
+func TestDiffSerializationErrorDoesNotExposeWriterDetails(t *testing.T) {
+	err := writeDiffDocument(diffErrorWriter{}, &document.Node{Kind: document.String, Text: "safe"})
+	if err != errDiffSnapshotUnavailable || strings.Contains(err.Error(), "CANARY") {
+		t.Fatalf("serialization error = %v", err)
+	}
+}
+
+type diffErrorWriter struct{}
+
+func (diffErrorWriter) Write([]byte) (int, error) {
+	return 0, errors.New("CANARY_WRITER_ERROR")
+}

internal/engine/diff_sensitivity.go

@@ -0,0 +1,48 @@
+package engine
+
+type diffSensitivity struct {
+	prefixes pathPrefixIndex
+	pointers []string
+}
+
+func newDiffSensitivity(left, right map[string]bool) *diffSensitivity {
+	sensitivity := &diffSensitivity{}
+	seen := make(map[string]struct{}, len(left)+len(right))
+	for _, classifications := range []map[string]bool{left, right} {
+		for pointer, classified := range classifications {
+			if !classified {
+				continue
+			}
+			if _, found := seen[pointer]; found {
+				continue
+			}
+			seen[pointer] = struct{}{}
+			id := len(sensitivity.pointers)
+			sensitivity.pointers = append(sensitivity.pointers, pointer)
+			sensitivity.prefixes.insert(pointer, id)
+		}
+	}
+	return sensitivity
+}
+
+func (sensitivity *diffSensitivity) isSensitiveAtOrAbove(pointer string) bool {
+	if sensitivity == nil {
+		return false
+	}
+	found := false
+	sensitivity.prefixes.visitAncestors(pointer, func(int) { found = true })
+	return found
+}
+
+func (sensitivity *diffSensitivity) hasStrictDescendant(pointer string) bool {
+	if sensitivity == nil {
+		return false
+	}
+	found := false
+	sensitivity.prefixes.visitDescendants(pointer, func(id int) {
+		if sensitivity.pointers[id] != pointer {
+			found = true
+		}
+	})
+	return found
+}

internal/engine/diff_sensitivity_test.go

@@ -0,0 +1,90 @@
+package engine
+
+import (
+	"maps"
+	"testing"
+)
+
+func TestDiffSensitivityCombinesClassificationsWithoutMutation(t *testing.T) {
+	left := map[string]bool{
+		"/a":                  true,
+		"/a/b":                true,
+		"/historical/removed": true,
+		"/cross~1key":         true,
+		"/literal~01slash":    true,
+		"/items/01":           true,
+		"/items/+1":           false,
+		"/false-left":         false,
+	}
+	right := map[string]bool{
+		"/a/b":         true,
+		"/right-only":  true,
+		"/false-right": false,
+	}
+	leftBefore, rightBefore := maps.Clone(left), maps.Clone(right)
+	sensitivity := newDiffSensitivity(left, right)
+
+	for _, test := range []struct {
+		pointer string
+		want    bool
+	}{
+		{pointer: "/a", want: true},
+		{pointer: "/a/b", want: true},
+		{pointer: "/a/b/c", want: true},
+		{pointer: "/ab", want: false},
+		{pointer: "/ab/c", want: false},
+		{pointer: "/historical/removed", want: true},
+		{pointer: "/historical/removed/child", want: true},
+		{pointer: "/historical", want: false},
+		{pointer: "/right-only", want: true},
+		{pointer: "/false-left", want: false},
+		{pointer: "/false-right", want: false},
+		{pointer: "/cross~1key", want: true},
+		{pointer: "/cross/key", want: false},
+		{pointer: "/literal~01slash", want: true},
+		{pointer: "/literal~1slash", want: false},
+		{pointer: "/items/01", want: true},
+		{pointer: "/items/1", want: false},
+		{pointer: "/items/+1", want: false},
+	} {
+		if got := sensitivity.isSensitiveAtOrAbove(test.pointer); got != test.want {
+			t.Errorf("isSensitiveAtOrAbove(%q) = %v, want %v", test.pointer, got, test.want)
+		}
+	}
+
+	for _, test := range []struct {
+		pointer string
+		want    bool
+	}{
+		{pointer: "", want: true},
+		{pointer: "/a", want: true},
+		{pointer: "/a/b", want: false},
+		{pointer: "/ab", want: false},
+		{pointer: "/historical", want: true},
+		{pointer: "/right-only", want: false},
+		{pointer: "/cross~1key", want: false},
+	} {
+		if got := sensitivity.hasStrictDescendant(test.pointer); got != test.want {
+			t.Errorf("hasStrictDescendant(%q) = %v, want %v", test.pointer, got, test.want)
+		}
+	}
+
+	if !maps.Equal(left, leftBefore) || !maps.Equal(right, rightBefore) {
+		t.Fatal("classification maps were modified")
+	}
+}
+
+func TestDiffSensitivityDistinguishesRootAndEmptyKey(t *testing.T) {
+	root := newDiffSensitivity(map[string]bool{"": true}, nil)
+	if !root.isSensitiveAtOrAbove("") || !root.isSensitiveAtOrAbove("/empty") ||
+		root.hasStrictDescendant("") {
+		t.Fatal("root classification did not remain distinct from strict descendants")
+	}
+
+	emptyKey := newDiffSensitivity(map[string]bool{"/": true}, nil)
+	if emptyKey.isSensitiveAtOrAbove("") || !emptyKey.isSensitiveAtOrAbove("/") ||
+		emptyKey.isSensitiveAtOrAbove("/other") || !emptyKey.hasStrictDescendant("") ||
+		emptyKey.hasStrictDescendant("/") {
+		t.Fatal("empty-key classification was treated as the root or an unrelated path")
+	}
+}

internal/engine/diff_snapshot.go

@@ -0,0 +1,195 @@
+package engine
+
+import (
+	"sort"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type pairedDiffSnapshot struct {
+	left             *document.Node
+	right            *document.Node
+	leftRedacted     bool
+	rightRedacted    bool
+	requiresFallback bool
+	hiddenChanged    bool
+}
+
+func preparePairedDiffSnapshot(left, right *document.Node, path Path, sensitivity *diffSensitivity) (pairedDiffSnapshot, bool) {
+	paired, supported := preparePairedDiffNodes(left, right, path, sensitivity)
+	if !supported || !paired.requiresFallback {
+		return paired, supported
+	}
+	return pairedDiffSnapshot{
+		left:          redactedDiffNode(left),
+		right:         redactedDiffNode(right),
+		leftRedacted:  left != nil,
+		rightRedacted: right != nil,
+		hiddenChanged: true,
+	}, true
+}
+
+func preparePairedDiffNodes(left, right *document.Node, path Path, sensitivity *diffSensitivity) (pairedDiffSnapshot, bool) {
+	if left == nil && right == nil {
+		return pairedDiffSnapshot{}, true
+	}
+	if sensitivity.isSensitiveAtOrAbove(path.String()) {
+		return pairedDiffSnapshot{
+			left:          redactedDiffNode(left),
+			right:         redactedDiffNode(right),
+			leftRedacted:  left != nil,
+			rightRedacted: right != nil,
+			hiddenChanged: !configurationValuesEqual(left, right),
+		}, true
+	}
+	if left != nil && right != nil && left.Kind != right.Kind {
+		if sensitivity.hasStrictDescendant(path.String()) {
+			return pairedDiffSnapshot{requiresFallback: true}, true
+		}
+		return pairedDiffSnapshot{left: left, right: right}, true
+	}
+
+	kind := document.Null
+	if left != nil {
+		kind = left.Kind
+	} else if right != nil {
+		kind = right.Kind
+	}
+	switch kind {
+	case document.Mapping:
+		return preparePairedDiffMapping(left, right, path, sensitivity)
+	case document.Sequence:
+		return preparePairedDiffSequence(left, right, path, sensitivity)
+	default:
+		return pairedDiffSnapshot{left: left, right: right}, true
+	}
+}
+
+func preparePairedDiffMapping(left, right *document.Node, path Path, sensitivity *diffSensitivity) (pairedDiffSnapshot, bool) {
+	leftEntries := diffMappingEntries(left)
+	rightEntries := diffMappingEntries(right)
+	keys := make([]string, 0, len(leftEntries)+len(rightEntries))
+	seen := make(map[string]struct{}, len(leftEntries)+len(rightEntries))
+	for key := range leftEntries {
+		seen[key] = struct{}{}
+		keys = append(keys, key)
+	}
+	for key := range rightEntries {
+		if _, found := seen[key]; found {
+			continue
+		}
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+
+	var leftCopy, rightCopy *document.Node
+	if left != nil {
+		copy := *left
+		copy.Mapping = make([]document.Entry, 0, len(left.Mapping))
+		leftCopy = &copy
+	}
+	if right != nil {
+		copy := *right
+		copy.Mapping = make([]document.Entry, 0, len(right.Mapping))
+		rightCopy = &copy
+	}
+	redacted := pairedDiffSnapshot{}
+	for _, key := range keys {
+		leftEntry, leftPresent := leftEntries[key]
+		rightEntry, rightPresent := rightEntries[key]
+		var leftChild, rightChild *document.Node
+		if leftPresent {
+			leftChild = leftEntry.Value
+		}
+		if rightPresent {
+			rightChild = rightEntry.Value
+		}
+		child, supported := preparePairedDiffNodes(leftChild, rightChild, path.Key(key), sensitivity)
+		if !supported {
+			return pairedDiffSnapshot{}, false
+		}
+		if child.requiresFallback {
+			return pairedDiffSnapshot{requiresFallback: true}, true
+		}
+		if leftPresent {
+			leftEntry.Value = child.left
+			leftCopy.Mapping = append(leftCopy.Mapping, leftEntry)
+		}
+		if rightPresent {
+			rightEntry.Value = child.right
+			rightCopy.Mapping = append(rightCopy.Mapping, rightEntry)
+		}
+		redacted.leftRedacted = redacted.leftRedacted || child.leftRedacted
+		redacted.rightRedacted = redacted.rightRedacted || child.rightRedacted
+		redacted.hiddenChanged = redacted.hiddenChanged || child.hiddenChanged
+	}
+	redacted.left = leftCopy
+	redacted.right = rightCopy
+	return redacted, true
+}
+
+func preparePairedDiffSequence(left, right *document.Node, path Path, sensitivity *diffSensitivity) (pairedDiffSnapshot, bool) {
+	var leftCopy, rightCopy *document.Node
+	if left != nil {
+		copy := *left
+		copy.Sequence = make([]*document.Node, len(left.Sequence))
+		leftCopy = &copy
+	}
+	if right != nil {
+		copy := *right
+		copy.Sequence = make([]*document.Node, len(right.Sequence))
+		rightCopy = &copy
+	}
+	length := 0
+	if left != nil {
+		length = len(left.Sequence)
+	}
+	if right != nil && len(right.Sequence) > length {
+		length = len(right.Sequence)
+	}
+	result := pairedDiffSnapshot{left: leftCopy, right: rightCopy}
+	for index := 0; index < length; index++ {
+		var leftChild, rightChild *document.Node
+		if left != nil && index < len(left.Sequence) {
+			leftChild = left.Sequence[index]
+		}
+		if right != nil && index < len(right.Sequence) {
+			rightChild = right.Sequence[index]
+		}
+		child, supported := preparePairedDiffNodes(leftChild, rightChild, path.Index(index), sensitivity)
+		if !supported {
+			return pairedDiffSnapshot{}, false
+		}
+		if child.requiresFallback {
+			return pairedDiffSnapshot{requiresFallback: true}, true
+		}
+		if left != nil && index < len(left.Sequence) {
+			leftCopy.Sequence[index] = child.left
+		}
+		if right != nil && index < len(right.Sequence) {
+			rightCopy.Sequence[index] = child.right
+		}
+		result.leftRedacted = result.leftRedacted || child.leftRedacted
+		result.rightRedacted = result.rightRedacted || child.rightRedacted
+		result.hiddenChanged = result.hiddenChanged || child.hiddenChanged
+	}
+	return result, true
+}
+
+func diffMappingEntries(node *document.Node) map[string]document.Entry {
+	entries := make(map[string]document.Entry)
+	if node == nil {
+		return entries
+	}
+	for _, entry := range node.Mapping {
+		entries[entry.Key] = entry
+	}
+	return entries
+}
+
+func redactedDiffNode(node *document.Node) *document.Node {
+	if node == nil {
+		return nil
+	}
+	return &document.Node{Kind: document.String, Text: "[REDACTED]"}
+}

internal/engine/diff_snapshot_test.go

@@ -0,0 +1,351 @@
+package engine
+
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestPreparePairedDiffSnapshotRedactsAlignedMappingFromEitherSide(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "credential: alpha\npublic: before\n")
+	right := parseCompositionDocument(t, "right", "credential: bravo\npublic: after\n")
+	sensitivity := newDiffSensitivity(map[string]bool{"/credential": true}, nil)
+
+	paired, supported := preparePairedDiffSnapshot(left, right, Path{}, sensitivity)
+	if !supported {
+		t.Fatal("compatible mappings were not supported")
+	}
+	if !paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("redacted flags = %v, %v; want true on both sides", paired.leftRedacted, paired.rightRedacted)
+	}
+	assertDiffStringNode(t, mappingValue(t, paired.left, "credential"), "[REDACTED]")
+	assertDiffStringNode(t, mappingValue(t, paired.right, "credential"), "[REDACTED]")
+	assertDiffStringNode(t, mappingValue(t, paired.left, "public"), "before")
+	assertDiffStringNode(t, mappingValue(t, paired.right, "public"), "after")
+	assertDiffStringNode(t, mappingValue(t, left, "credential"), "alpha")
+	assertDiffStringNode(t, mappingValue(t, right, "credential"), "bravo")
+	if paired.left == left || paired.right == right || &paired.left.Mapping[0] == &left.Mapping[0] {
+		t.Fatal("safe mapping containers or entries share mutable storage with retained trees")
+	}
+}
+
+func TestPreparePairedDiffSnapshotPreservesSequencePositionsAndLengths(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "items: [left-public, hidden, old-tail]\n")
+	right := parseCompositionDocument(t, "right", "items: [right-public, hidden, old-tail, extra]\n")
+	sensitivity := newDiffSensitivity(nil, map[string]bool{"/items/1": true})
+
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "items"), mappingValue(t, right, "items"), Keys("items"), sensitivity,
+	)
+	if !supported {
+		t.Fatal("compatible sequences were not supported")
+	}
+	if !paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("redacted flags = %v, %v; want true on both sides", paired.leftRedacted, paired.rightRedacted)
+	}
+	if len(paired.left.Sequence) != 3 || len(paired.right.Sequence) != 4 {
+		t.Fatalf("sequence lengths = %d, %d; want 3, 4", len(paired.left.Sequence), len(paired.right.Sequence))
+	}
+	assertDiffStringNode(t, paired.left.Sequence[0], "left-public")
+	assertDiffStringNode(t, paired.right.Sequence[0], "right-public")
+	assertDiffStringNode(t, paired.left.Sequence[1], "[REDACTED]")
+	assertDiffStringNode(t, paired.right.Sequence[1], "[REDACTED]")
+	assertDiffStringNode(t, paired.right.Sequence[3], "extra")
+	assertDiffStringNode(t, mappingValue(t, left, "items").Sequence[1], "hidden")
+	if &paired.left.Sequence[0] == &mappingValue(t, left, "items").Sequence[0] {
+		t.Fatal("safe sequence shares mutable storage with retained tree")
+	}
+}
+
+func TestPreparePairedDiffSnapshotMasksSensitiveDescendantsOfAbsentSubtrees(t *testing.T) {
+	tests := []struct {
+		name        string
+		left, right string
+		absentSide  string
+	}{
+		{
+			name:       "removed subtree",
+			left:       "removed: {token: old-secret, public: retained}\n",
+			right:      "other: value\n",
+			absentSide: "right",
+		},
+		{
+			name:       "added subtree",
+			left:       "other: value\n",
+			right:      "added: {token: new-secret, public: retained}\n",
+			absentSide: "left",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			left := parseCompositionDocument(t, "left", test.left)
+			right := parseCompositionDocument(t, "right", test.right)
+			paired, supported := preparePairedDiffSnapshot(
+				mappingValueIfPresent(left, "removed", "added"),
+				mappingValueIfPresent(right, "removed", "added"),
+				Keys("subtree"),
+				newDiffSensitivity(map[string]bool{"/subtree/token": true}, nil),
+			)
+			if !supported {
+				t.Fatal("compatible subtree and absence were not supported")
+			}
+			if test.absentSide == "left" {
+				if paired.left != nil || paired.leftRedacted || !paired.rightRedacted {
+					t.Fatalf("paired absence/redaction = %#v", paired)
+				}
+				assertDiffStringNode(t, mappingValue(t, paired.right, "token"), "[REDACTED]")
+				assertDiffStringNode(t, mappingValue(t, paired.right, "public"), "retained")
+			} else {
+				if paired.right != nil || paired.rightRedacted || !paired.leftRedacted {
+					t.Fatalf("paired absence/redaction = %#v", paired)
+				}
+				assertDiffStringNode(t, mappingValue(t, paired.left, "token"), "[REDACTED]")
+				assertDiffStringNode(t, mappingValue(t, paired.left, "public"), "retained")
+			}
+		})
+	}
+}
+
+func TestPreparePairedDiffSnapshotMasksSelectedRootAndContainer(t *testing.T) {
+	tests := []struct {
+		name        string
+		left, right *document.Node
+		path        Path
+		selector    string
+	}{
+		{
+			name:     "root",
+			left:     parseCompositionDocument(t, "left", "token: old\n"),
+			right:    parseCompositionDocument(t, "right", "token: new\n"),
+			path:     Path{},
+			selector: "",
+		},
+		{
+			name:     "container",
+			left:     parseCompositionDocument(t, "left", "group: {public: old}\n"),
+			right:    parseCompositionDocument(t, "right", "group: {public: new}\n"),
+			path:     Keys("group"),
+			selector: "/group",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			paired, supported := preparePairedDiffSnapshot(
+				test.left, test.right, test.path,
+				newDiffSensitivity(map[string]bool{test.selector: true}, nil),
+			)
+			if !supported {
+				t.Fatal("selected nodes were not supported")
+			}
+			if !paired.leftRedacted || !paired.rightRedacted {
+				t.Fatalf("redacted flags = %v, %v; want true on both sides", paired.leftRedacted, paired.rightRedacted)
+			}
+			assertDiffStringNode(t, paired.left, "[REDACTED]")
+			assertDiffStringNode(t, paired.right, "[REDACTED]")
+		})
+	}
+}
+
+func TestPreparePairedDiffSnapshotDoesNotInterpretMarkerLookingInput(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "label: '[REDACTED]'\n")
+	right := parseCompositionDocument(t, "right", "label: '[REDACTED]'\n")
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "label"), mappingValue(t, right, "label"), Keys("label"),
+		newDiffSensitivity(nil, nil),
+	)
+	if !supported {
+		t.Fatal("ordinary scalar values were not supported")
+	}
+	if paired.leftRedacted || paired.rightRedacted {
+		t.Fatalf("redacted flags = %v, %v; want false", paired.leftRedacted, paired.rightRedacted)
+	}
+	if paired.left != mappingValue(t, left, "label") || paired.right != mappingValue(t, right, "label") {
+		t.Fatal("unselected scalar nodes should remain borrowed immutable nodes")
+	}
+	assertDiffStringNode(t, paired.left, "[REDACTED]")
+	assertDiffStringNode(t, paired.right, "[REDACTED]")
+}
+
+func TestPreparePairedDiffSnapshotMasksSensitiveStructuralReplacements(t *testing.T) {
+	tests := []struct {
+		name        string
+		left, right string
+		path        Path
+		selector    string
+	}{
+		{
+			name:     "mapping to scalar",
+			left:     "changed: {token: old-secret, public: old}\n",
+			right:    "changed: new-scalar\n",
+			path:     Keys("changed"),
+			selector: "/changed/token",
+		},
+		{
+			name:     "mapping to scalar with historical-only descendant",
+			left:     "changed: {public: old}\n",
+			right:    "changed: new-scalar\n",
+			path:     Keys("changed"),
+			selector: "/changed/removed/secret",
+		},
+		{
+			name:     "scalar to mapping",
+			left:     "changed: old-scalar\n",
+			right:    "changed: {token: new-secret, public: new}\n",
+			path:     Keys("changed"),
+			selector: "/changed/token",
+		},
+		{
+			name:     "mapping to sequence with numeric paths",
+			left:     "changed: {'0': {token: old-secret}}\n",
+			right:    "changed: [{token: new-secret}]\n",
+			path:     Keys("changed"),
+			selector: "/changed/0/token",
+		},
+		{
+			name:     "sequence to mapping with numeric paths",
+			left:     "changed: [{token: old-secret}]\n",
+			right:    "changed: {'0': {token: new-secret}}\n",
+			path:     Keys("changed"),
+			selector: "/changed/0/token",
+		},
+		{
+			name:     "null to container",
+			left:     "changed: null\n",
+			right:    "changed: {token: new-secret}\n",
+			path:     Keys("changed"),
+			selector: "/changed/token",
+		},
+		{
+			name:     "container to null",
+			left:     "changed: {token: old-secret}\n",
+			right:    "changed: null\n",
+			path:     Keys("changed"),
+			selector: "/changed/token",
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			leftRoot := parseCompositionDocument(t, "left", test.left)
+			rightRoot := parseCompositionDocument(t, "right", test.right)
+			left := mappingValue(t, leftRoot, test.path.segments[0].key)
+			right := mappingValue(t, rightRoot, test.path.segments[0].key)
+			paired, supported := preparePairedDiffSnapshot(
+				left, right, test.path, newDiffSensitivity(nil, map[string]bool{test.selector: true}),
+			)
+			assertWholeRegionFallback(t, paired, supported, left, right)
+		})
+	}
+}
+
+func TestPreparePairedDiffSnapshotMasksOriginalRegionForNestedSequenceMismatch(t *testing.T) {
+	leftRoot := parseCompositionDocument(t, "left", "items: [{branch: {token: old-secret, public: old}}]\n")
+	rightRoot := parseCompositionDocument(t, "right", "items: [{branch: new-scalar}]\n")
+	left := mappingValue(t, leftRoot, "items")
+	right := mappingValue(t, rightRoot, "items")
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Keys("items"), newDiffSensitivity(nil, map[string]bool{"/items/0/branch/token": true}),
+	)
+	assertWholeRegionFallback(t, paired, supported, left, right)
+}
+
+func TestPreparePairedDiffSnapshotKeepsPublicScalarsWithHistoricalDescendants(t *testing.T) {
+	leftRoot := parseCompositionDocument(t, "left", "changed: before\n")
+	rightRoot := parseCompositionDocument(t, "right", "changed: after\n")
+	left := mappingValue(t, leftRoot, "changed")
+	right := mappingValue(t, rightRoot, "changed")
+	paired, supported := preparePairedDiffSnapshot(
+		left, right, Keys("changed"), newDiffSensitivity(map[string]bool{"/changed/removed/secret": true}, nil),
+	)
+	if !supported || paired.leftRedacted || paired.rightRedacted || paired.hiddenChanged {
+		t.Fatalf("public scalar replacement = %#v, supported=%v", paired, supported)
+	}
+	assertDiffStringNode(t, paired.left, "before")
+	assertDiffStringNode(t, paired.right, "after")
+}
+
+func TestPreparePairedDiffSnapshotKeepsPublicContentAcrossAbsence(t *testing.T) {
+	right := parseCompositionDocument(t, "right", "branch: scalar\n")
+	paired, supported := preparePairedDiffSnapshot(
+		nil, right, Keys("added"), newDiffSensitivity(nil, map[string]bool{"/added/branch/secret": true}),
+	)
+	if !supported || paired.left != nil || paired.leftRedacted || paired.rightRedacted || paired.hiddenChanged {
+		t.Fatalf("public addition presence/redaction = %#v, supported=%v", paired, supported)
+	}
+	assertDiffStringNode(t, mappingValue(t, paired.right, "branch"), "scalar")
+}
+
+func TestPreparePairedDiffSnapshotPreservesPublicReplacementBesideSensitiveSibling(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "child: before\n")
+	right := &document.Node{Kind: document.String, Text: "after"}
+	paired, supported := preparePairedDiffSnapshot(
+		mappingValue(t, left, "child"), right, Keys("public"),
+		newDiffSensitivity(nil, map[string]bool{"/private/token": true}),
+	)
+	if !supported || paired.left != mappingValue(t, left, "child") || paired.right != right || paired.leftRedacted || paired.rightRedacted {
+		t.Fatalf("unrelated classification changed public replacement: %#v, supported=%v", paired, supported)
+	}
+}
+
+func TestPreparePairedDiffSnapshotTargetsRedactionAcrossAbsence(t *testing.T) {
+	left := parseCompositionDocument(t, "left", "items: []\n")
+	right := parseCompositionDocument(t, "right", "items: [{token: new-secret, public: visible}]\n")
+	leftItems := mappingValue(t, left, "items")
+	rightItems := mappingValue(t, right, "items")
+	paired, supported := preparePairedDiffSnapshot(
+		leftItems, rightItems, Keys("items"), newDiffSensitivity(nil, map[string]bool{"/items/0/token": true}),
+	)
+	if !supported || paired.left == nil || paired.right == nil || paired.leftRedacted || !paired.rightRedacted {
+		t.Fatalf("targeted sequence redaction = %#v, supported=%v", paired, supported)
+	}
+	if len(paired.left.Sequence) != 0 || len(paired.right.Sequence) != 1 {
+		t.Fatalf("sequence lengths = %d, %d; want 0, 1", len(paired.left.Sequence), len(paired.right.Sequence))
+	}
+	assertDiffStringNode(t, mappingValue(t, paired.right.Sequence[0], "token"), "[REDACTED]")
+	assertDiffStringNode(t, mappingValue(t, paired.right.Sequence[0], "public"), "visible")
+}
+
+func assertWholeRegionFallback(t *testing.T, paired pairedDiffSnapshot, supported bool, left, right *document.Node) {
+	t.Helper()
+	if !supported {
+		t.Fatal("sensitive incompatible shape was not handled")
+	}
+	if !paired.hiddenChanged {
+		t.Fatal("fallback for a changed reported region did not mark hidden change")
+	}
+	if (left == nil) != (paired.left == nil) || (right == nil) != (paired.right == nil) {
+		t.Fatalf("fallback changed root presence: %#v", paired)
+	}
+	if left != nil {
+		assertDiffStringNode(t, paired.left, "[REDACTED]")
+		if !paired.leftRedacted {
+			t.Fatal("present left region was not marked redacted")
+		}
+	}
+	if right != nil {
+		assertDiffStringNode(t, paired.right, "[REDACTED]")
+		if !paired.rightRedacted {
+			t.Fatal("present right region was not marked redacted")
+		}
+	}
+}
+
+func mappingValueIfPresent(node *document.Node, keys ...string) *document.Node {
+	if node == nil {
+		return nil
+	}
+	for _, key := range keys {
+		for _, entry := range node.Mapping {
+			if entry.Key == key {
+				return entry.Value
+			}
+		}
+	}
+	return nil
+}
+
+func assertDiffStringNode(t *testing.T, node *document.Node, want string) {
+	t.Helper()
+	if node == nil || node.Kind != document.String || node.Text != want {
+		t.Fatalf("node = %#v, want string %q", node, want)
+	}
+}

internal/engine/diff_test.go

@@ -0,0 +1,144 @@
+package engine
+
+import (
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestPrepareDiffRegionsUsesCanonicalConfigurationComparison(t *testing.T) {
+	tests := []struct {
+		name  string
+		left  string
+		right string
+		want  []expectedDiffRegion
+	}{
+		{
+			name:  "mapping order is ignored for equal values",
+			left:  "first: 1\nsecond: true\n",
+			right: "second: true\nfirst: 1\n",
+		},
+		{
+			name:  "addition removal null and empty values",
+			left:  "removed: null\n",
+			right: "added: null\nempty_map: {}\nempty_sequence: []\n",
+			want: []expectedDiffRegion{
+				{path: "/added", rightKind: document.Null, rightPresent: true},
+				{path: "/empty_map", rightKind: document.Mapping, rightPresent: true},
+				{path: "/empty_sequence", rightKind: document.Sequence, rightPresent: true},
+				{path: "/removed", leftKind: document.Null, leftPresent: true},
+			},
+		},
+		{
+			name:  "scalar kinds and atomic sequences",
+			left:  "scalar: 1\nitems: [one, two]\n",
+			right: "scalar: '1'\nitems: [one, changed]\n",
+			want: []expectedDiffRegion{
+				{path: "/items", leftKind: document.Sequence, rightKind: document.Sequence, leftPresent: true, rightPresent: true},
+				{path: "/scalar", leftKind: document.Number, rightKind: document.String, leftPresent: true, rightPresent: true},
+			},
+		},
+		{
+			name:  "escaped empty and numeric mapping keys sort by pointer",
+			left:  `"": old` + "\n'0': old\n'01': old\n'a/b': old\n'a~b': old\n",
+			right: `"": new` + "\n'0': new\n'01': new\n'a/b': new\n'a~b': new\n",
+			want: []expectedDiffRegion{
+				{path: "/", leftKind: document.String, rightKind: document.String, leftPresent: true, rightPresent: true},
+				{path: "/0", leftKind: document.String, rightKind: document.String, leftPresent: true, rightPresent: true},
+				{path: "/01", leftKind: document.String, rightKind: document.String, leftPresent: true, rightPresent: true},
+				{path: "/a~0b", leftKind: document.String, rightKind: document.String, leftPresent: true, rightPresent: true},
+				{path: "/a~1b", leftKind: document.String, rightKind: document.String, leftPresent: true, rightPresent: true},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			leftRoot := parseCompositionDocument(t, "left", test.left)
+			rightRoot := parseCompositionDocument(t, "right", test.right)
+			regions, err := prepareDiffRegions(diffTestInspection(leftRoot), diffTestInspection(rightRoot))
+			if err != nil {
+				t.Fatal(err)
+			}
+			if len(regions) != len(test.want) {
+				t.Fatalf("regions = %#v, want %#v", regions, test.want)
+			}
+			for index, want := range test.want {
+				region := regions[index]
+				if region.path.String() != want.path {
+					t.Fatalf("region[%d].path = %q, want %q", index, region.path.String(), want.path)
+				}
+				assertDiffRegionNode(t, "left", region.left, want.leftPresent, want.leftKind)
+				assertDiffRegionNode(t, "right", region.right, want.rightPresent, want.rightKind)
+			}
+		})
+	}
+}
+
+func TestPrepareDiffRegionsHandlesRootReplacementAndBorrowsNodes(t *testing.T) {
+	leftRoot := &document.Node{Kind: document.String, Text: "before"}
+	rightRoot := &document.Node{Kind: document.Mapping, Mapping: []document.Entry{{
+		Key: "value", Value: &document.Node{Kind: document.Number, Text: "1"},
+	}}}
+	regions, err := prepareDiffRegions(diffTestInspection(leftRoot), diffTestInspection(rightRoot))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(regions) != 1 || regions[0].path.String() != "" || regions[0].left != leftRoot || regions[0].right != rightRoot {
+		t.Fatalf("root region = %#v", regions)
+	}
+
+	leftRoot = parseCompositionDocument(t, "left-child", "value: before\n")
+	rightRoot = parseCompositionDocument(t, "right-child", "value: after\n")
+	regions, err = prepareDiffRegions(diffTestInspection(leftRoot), diffTestInspection(rightRoot))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(regions) != 1 || regions[0].left != mappingValue(t, leftRoot, "value") || regions[0].right != mappingValue(t, rightRoot, "value") {
+		t.Fatalf("child nodes were not borrowed from the frozen trees: %#v", regions)
+	}
+}
+
+func TestPrepareDiffRegionsRejectsUnavailableInspections(t *testing.T) {
+	valid := diffTestInspection(&document.Node{Kind: document.String, Text: "safe"})
+	tests := []struct {
+		name        string
+		left, right *Inspection
+	}{
+		{name: "nil left", left: nil, right: valid},
+		{name: "nil right", left: valid, right: nil},
+		{name: "missing left tree", left: &Inspection{}, right: valid},
+		{name: "missing right tree", left: valid, right: &Inspection{}},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			regions, err := prepareDiffRegions(test.left, test.right)
+			if err == nil || regions != nil {
+				t.Fatalf("regions, err = %#v, %v", regions, err)
+			}
+		})
+	}
+}
+
+type expectedDiffRegion struct {
+	path                      string
+	leftKind, rightKind       document.Kind
+	leftPresent, rightPresent bool
+}
+
+func diffTestInspection(root *document.Node) *Inspection {
+	return &Inspection{final: root, immutableLookup: buildDocumentLookup(root)}
+}
+
+func assertDiffRegionNode(t *testing.T, side string, node *document.Node, wantPresent bool, wantKind document.Kind) {
+	t.Helper()
+	if !wantPresent {
+		if node != nil {
+			t.Fatalf("%s node = %#v, want absence", side, node)
+		}
+		return
+	}
+	if node == nil || node.Kind != wantKind {
+		t.Fatalf("%s node = %#v, want kind %v", side, node, wantKind)
+	}
+}

internal/engine/document_compare.go

@@ -24,12 +24,23 @@ type documentDelta struct {
 func compareConfigurationValues(before, after *document.Node) []documentDelta {
 	var deltas []documentDelta
 	compareConfigurationValue(before, after, Path{}, &deltas)
-	slices.SortFunc(deltas, func(left, right documentDelta) int {
-		if order := cmp.Compare(left.path.String(), right.path.String()); order != 0 {
+	type orderedDelta struct {
+		delta   documentDelta
+		pointer string
+	}
+	ordered := make([]orderedDelta, len(deltas))
+	for i, delta := range deltas {
+		ordered[i] = orderedDelta{delta: delta, pointer: delta.path.String()}
+	}
+	slices.SortFunc(ordered, func(left, right orderedDelta) int {
+		if order := cmp.Compare(left.pointer, right.pointer); order != 0 {
 			return order
 		}
-		return int(left.kind) - int(right.kind)
+		return int(left.delta.kind) - int(right.delta.kind)
 	})
+	for i, item := range ordered {
+		deltas[i] = item.delta
+	}
 	return deltas
 }
 

internal/engine/document_lookup.go

@@ -0,0 +1,54 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+// documentLookup contains read-only mapping positions for a completed document.
+type documentLookup struct {
+	byNode map[*document.Node]map[string]int
+}
+
+func buildDocumentLookup(roots ...*document.Node) *documentLookup {
+	lookup := &documentLookup{byNode: make(map[*document.Node]map[string]int)}
+	visited := make(map[*document.Node]struct{})
+	var visit func(*document.Node)
+	visit = func(node *document.Node) {
+		if node == nil {
+			return
+		}
+		if _, ok := visited[node]; ok {
+			return
+		}
+		visited[node] = struct{}{}
+		switch node.Kind {
+		case document.Mapping:
+			entries := make(map[string]int, len(node.Mapping))
+			for index, entry := range node.Mapping {
+				if _, exists := entries[entry.Key]; !exists {
+					entries[entry.Key] = index
+				}
+				visit(entry.Value)
+			}
+			lookup.byNode[node] = entries
+		case document.Sequence:
+			for _, child := range node.Sequence {
+				visit(child)
+			}
+		}
+	}
+	for _, root := range roots {
+		visit(root)
+	}
+	return lookup
+}
+
+func (lookup *documentLookup) find(node *document.Node, key string) int {
+	if lookup == nil {
+		return -1
+	}
+	if entries := lookup.byNode[node]; entries != nil {
+		if index, ok := entries[key]; ok {
+			return index
+		}
+	}
+	return -1
+}

internal/engine/document_walk.go

@@ -31,6 +31,10 @@ func indexDocument(node *document.Node, path Path, index map[string]*document.No
 }
 
 func documentAtPath(root *document.Node, path Path) (*document.Node, bool) {
+	return documentAtPathWithLookup(root, path, nil)
+}
+
+func documentAtPathWithLookup(root *document.Node, path Path, lookup *documentLookup) (*document.Node, bool) {
 	if root == nil || !path.valid() {
 		return nil, false
 	}
@@ -41,17 +45,21 @@ func documentAtPath(root *document.Node, path Path) (*document.Node, bool) {
 			if segment.kind == pathSegmentIndex {
 				return nil, false
 			}
-			found := false
-			for _, entry := range current.Mapping {
+			entryIndex := -1
+			if lookup != nil {
+				entryIndex = lookup.find(current, segment.key)
+			} else {
+				for index, entry := range current.Mapping {
 					if entry.Key == segment.key {
-					current = entry.Value
-					found = true
+						entryIndex = index
 						break
 					}
 				}
-			if !found {
+			}
+			if entryIndex < 0 || entryIndex >= len(current.Mapping) {
 				return nil, false
 			}
+			current = current.Mapping[entryIndex].Value
 		case document.Sequence:
 			index, ok := schemaIndex(segment)
 			if !ok || index >= len(current.Sequence) {

internal/engine/explain.go

@@ -40,7 +40,7 @@ func (r *Inspection) Explain(path Path) (Explanation, bool) {
 	if r == nil || r.final == nil || r.baseline == nil || !r.metadata.hasHistoricalPath(path) {
 		return Explanation{}, false
 	}
-	explanation := Explanation{Path: path.clone(), Effective: valueSnapshot(r.final, path, r.secrets)}
+	explanation := Explanation{Path: path.clone(), Effective: valueSnapshotWithLookup(r.final, path, r.secrets, r.immutableLookup)}
 	if info, ok := r.metadata.Lookup(path); ok {
 		explanation.Origin = cloneOrigin(info.Origin)
 	}
@@ -51,13 +51,14 @@ func (r *Inspection) Explain(path Path) (Explanation, bool) {
 			After: valueSnapshot(working, path, r.secrets), Origin: Origin{Layer: LayerDefault},
 		})
 	}
+	replayIndex := new(mappingIndex)
 	for _, application := range r.applications {
-		operationPath, relevant := applicationOperationPath(application, path)
+		operationPath, relevant := applicationOperationPath(application, path, r.immutableLookup)
 		var before ValueSnapshot
 		if relevant {
 			before = valueSnapshot(working, path, r.secrets)
 		}
-		applyHistoryApplication(&working, application)
+		applyHistoryApplication(&working, application, replayIndex)
 		if !relevant {
 			continue
 		}
@@ -68,28 +69,29 @@ func (r *Inspection) Explain(path Path) (Explanation, bool) {
 		explanation.History = append(explanation.History, ValueChange{
 			Before: before,
 			After:  after,
-			Origin: historyApplicationOrigin(application, operationPath),
+			Origin: historyApplicationOrigin(application, operationPath, r.immutableLookup),
 		})
 	}
+	appendInheritanceExplanation(&explanation, &working, path, r.inheritance, r.secrets)
 
 	// Intermediate input replacements do not permanently suppress defaults:
 	// decoding applies them to the final input tree. Show that reconciliation
 	// explicitly, including canonical scalar conversions, before domain changes.
 	replayed, _ := documentAtPath(working, path)
-	decoded, _ := documentAtPath(r.input, path)
+	decoded, _ := documentAtPathWithLookup(r.input, path, r.immutableLookup)
 	if !configurationValuesEqual(replayed, decoded) {
 		origin := cloneOrigin(r.inputOrigins[path.String()])
 		origin.Operation = ChangeDecoding
 		explanation.History = append(explanation.History, ValueChange{
 			Before:  valueSnapshot(working, path, r.secrets),
-			After:   valueSnapshot(r.input, path, r.secrets),
+			After:   valueSnapshotWithLookup(r.input, path, r.secrets, r.immutableLookup),
 			Origin:  origin,
 			Reasons: []string{"applied retained defaults and canonical typed decoding"},
 		})
 	}
 
 	working = document.Clone(r.input)
-	for _, snapshot := range coalescedResolutionSnapshots(r.resolution) {
+	for _, snapshot := range r.resolutionReplay {
 		relevant := pathsIntersect(snapshot.change.Path, path)
 		var before ValueSnapshot
 		if relevant {

internal/engine/explain_render.go

@@ -23,10 +23,14 @@ func (r *Inspection) WriteExplanation(output io.Writer, path Path) error {
 		return errExplanationUnknownPath
 	}
 	var rendered strings.Builder
+	labels := make(map[string]string, len(r.sources))
+	for _, source := range r.sources {
+		labels[source.ID] = source.Label
+	}
 	rendered.WriteString("path: ")
 	rendered.WriteString(strconv.Quote(explanation.Path.String()))
 	rendered.WriteByte('\n')
-	writeExplanationOrigin(&rendered, "effective source", explanation.Origin, r.sources)
+	writeExplanationOrigin(&rendered, "effective source", explanation.Origin, labels)
 	writeExplanationSnapshot(&rendered, "effective (canonical typed)", explanation.Effective, 0)
 	if explanation.SecretReference != nil {
 		rendered.WriteString("secret reference: ")
@@ -38,12 +42,14 @@ func (r *Inspection) WriteExplanation(output io.Writer, path Path) error {
 		rendered.WriteString("  - representation: ")
 		if change.Origin.Operation == ChangeDecoding {
 			rendered.WriteString("input to canonical typed\n")
-		} else if change.Origin.Layer == LayerBase || change.Origin.Layer == LayerOverlay || change.Origin.Layer == LayerCLI {
+		} else if change.Origin.Operation == ChangeInheritance {
+			rendered.WriteString("structural inheritance\n")
+		} else if change.Origin.Layer == LayerBase || change.Origin.Layer == LayerUnderlay || change.Origin.Layer == LayerOverlay || change.Origin.Layer == LayerCLI {
 			rendered.WriteString("policy-validated input\n")
 		} else {
 			rendered.WriteString("canonical typed\n")
 		}
-		writeExplanationOrigin(&rendered, "    source", change.Origin, r.sources)
+		writeExplanationOrigin(&rendered, "    source", change.Origin, labels)
 		writeExplanationSnapshot(&rendered, "    before", change.Before, 4)
 		writeExplanationSnapshot(&rendered, "    after", change.After, 4)
 		for _, reason := range change.Reasons {
@@ -65,7 +71,7 @@ func (r *Inspection) WriteExplanation(output io.Writer, path Path) error {
 	return err
 }
 
-func writeExplanationOrigin(output *strings.Builder, label string, origin Origin, sources []SourceInfo) {
+func writeExplanationOrigin(output *strings.Builder, label string, origin Origin, labels map[string]string) {
 	output.WriteString(label)
 	output.WriteString(": layer=")
 	output.WriteString(strconv.Quote(string(origin.Layer)))
@@ -77,7 +83,7 @@ func writeExplanationOrigin(output *strings.Builder, label string, origin Origin
 	for _, location := range origin.Locations {
 		output.WriteString("      location: source=")
 		output.WriteString(strconv.Quote(location.SourceID))
-		if label := sourceLabel(sources, location.SourceID); label != "" {
+		if label := labels[location.SourceID]; label != "" {
 			output.WriteString(" label=")
 			output.WriteString(strconv.Quote(label))
 		}
@@ -87,15 +93,15 @@ func writeExplanationOrigin(output *strings.Builder, label string, origin Origin
 		output.WriteString(strconv.Itoa(location.Column))
 		output.WriteByte('\n')
 	}
+	for _, step := range origin.Inheritance {
+		output.WriteString("      inherited: from=")
+		output.WriteString(strconv.Quote(step.From.String()))
+		output.WriteString(" to=")
+		output.WriteString(strconv.Quote(step.To.String()))
+		output.WriteString(" reference=")
+		output.WriteString(strconv.Quote(step.Reference.String()))
+		output.WriteByte('\n')
 	}
-
-func sourceLabel(sources []SourceInfo, sourceID string) string {
-	for _, source := range sources {
-		if source.ID == sourceID {
-			return source.Label
-		}
-	}
-	return ""
 }
 
 func writeExplanationSnapshot(output *strings.Builder, label string, snapshot ValueSnapshot, indent int) {

internal/engine/explain_test.go

@@ -10,6 +10,7 @@ import (
 	"path/filepath"
 	"reflect"
 	"strings"
+	"sync"
 	"testing"
 
 	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
@@ -520,3 +521,95 @@ func TestExplanationReplaysResolutionAdditionsAndOverlappingIntent(t *testing.T)
 		})
 	}
 }
+
+func TestExplanationReplayDescriptorsRemainDetachedAcrossQueries(t *testing.T) {
+	type config struct {
+		Count int `yaml:"count"`
+	}
+	result, err := testLoad(Bytes("resolution-explanation.yml", []byte("count: 1\n")), Options[config]{
+		Definition: Definition[config]{Resolve: func(value *config, resolution *Resolution) error {
+			value.Count = 2
+			resolution.Record(Keys("count"), ChangeNormalization, "normalized", Keys("count"))
+			return nil
+		}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	first, ok := result.Explain(Keys("count"))
+	if !ok {
+		t.Fatal("missing explanation")
+	}
+	var firstResolution *ValueChange
+	for index := range first.History {
+		if first.History[index].Origin.Layer == LayerResolution {
+			firstResolution = &first.History[index]
+			break
+		}
+	}
+	if firstResolution == nil || len(firstResolution.Reasons) != 1 || len(firstResolution.Dependencies) != 1 {
+		t.Fatalf("resolution history = %#v", first.History)
+	}
+	firstResolution.Reasons[0] = "mutated"
+	firstResolution.Dependencies[0].segments[0].key = "mutated"
+
+	second, ok := result.Explain(Keys("count"))
+	if !ok {
+		t.Fatal("missing repeated explanation")
+	}
+	for _, change := range second.History {
+		if change.Origin.Layer == LayerResolution {
+			if !reflect.DeepEqual(change.Reasons, []string{"normalized"}) || len(change.Dependencies) != 1 || change.Dependencies[0].String() != "/count" {
+				t.Fatalf("repeated explanation observed caller mutation: %#v", change)
+			}
+			return
+		}
+	}
+	t.Fatalf("resolution change missing from repeated explanation: %#v", second.History)
+}
+
+func TestConcurrentReadOnlyInspectionQueries(t *testing.T) {
+	type config struct {
+		Group struct {
+			Value string `yaml:"value"`
+		} `yaml:"group"`
+	}
+	result, err := testLoad(Bytes("concurrent-inspection.yml", []byte("group: {value: input}\n")), Options[config]{
+		Definition: Definition[config]{Resolve: func(value *config, resolution *Resolution) error {
+			value.Group.Value = "resolved"
+			resolution.Record(Keys("group", "value"), ChangeNormalization, "normalized", Keys("group", "value"))
+			return nil
+		}},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	paths := []Path{Keys("group"), Keys("group", "value")}
+	var wait sync.WaitGroup
+	errors := make(chan string, 8)
+	for worker := 0; worker < cap(errors); worker++ {
+		wait.Add(1)
+		go func() {
+			defer wait.Done()
+			for iteration := 0; iteration < 8; iteration++ {
+				for _, path := range paths {
+					explanation, found := result.Explain(path)
+					if !found || !explanation.Effective.Present {
+						errors <- "missing effective explanation for " + path.String()
+						return
+					}
+					metadata := result.Metadata()
+					if _, found := metadata.Lookup(path); !found || len(metadata.History(path)) == 0 {
+						errors <- "missing metadata for " + path.String()
+						return
+					}
+				}
+			}
+		}()
+	}
+	wait.Wait()
+	close(errors)
+	for message := range errors {
+		t.Error(message)
+	}
+}

internal/engine/filesystem.go

@@ -1,12 +1,21 @@
 package engine
 
 import (
+	"io/fs"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 )
 
 func validateFilesystemContext(source Source, context *FilesystemContext, main parsedSource) *Diagnostic {
+	if nativeContext(source, context) {
+		access, basePath, mainPath, diagnostic := nativeFilesystemAccess(source, context)
+		if diagnostic != nil {
+			return diagnostic
+		}
+		return validateNativeDirectories(access, basePath, mainPath)
+	}
 	rootPath, _, _, diagnostic := importFilesystem(source, context, main)
 	if diagnostic != nil {
 		return diagnostic
@@ -29,20 +38,87 @@ func filesystemOptionDiagnostics(source Source, context *FilesystemContext) []Di
 	case sourceNone:
 		return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "NoSource cannot use filesystem context")}
 	case sourceFile:
+		if context.FS != nil {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "host file sources cannot use a native filesystem context")}
+		}
 		if context.BaseDir != "" {
 			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "file sources determine their own import base directory")}
 		}
-	case sourceBytes, sourceReader:
+	case sourceFilesystem:
+		if context.FS == nil || context.FS.namespace == nil || source.namespace == nil || context.FS.namespace.token != source.namespace.token {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "filesystem file context must use the source namespace")}
+		}
+		if context.BaseDir != "" {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "file sources determine their own import base directory")}
+		}
+		if context.Root != "" && !fs.ValidPath(context.Root) {
+			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "filesystem root must be a slash-relative valid path")}
+		}
+	case sourceBytes, sourceReader, sourceMapping:
 		if context.BaseDir == "" || context.Root == "" {
 			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "byte and reader filesystem context requires base and root directories")}
 		}
-		if !filepath.IsAbs(context.BaseDir) || !filepath.IsAbs(context.Root) {
+		if context.FS != nil {
+			if context.FS.namespace == nil || !fs.ValidPath(context.BaseDir) || !fs.ValidPath(context.Root) {
+				return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "filesystem base and root must be slash-relative valid paths")}
+			}
+		} else if !filepath.IsAbs(context.BaseDir) || !filepath.IsAbs(context.Root) {
 			return []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "byte and reader filesystem base and root directories must be absolute")}
 		}
 	}
 	return nil
 }
 
+func nativeContext(source Source, context *FilesystemContext) bool {
+	return source.kind == sourceFilesystem || context != nil && context.FS != nil
+}
+
+// nativeFilesystemAccess resolves namespace context without opening caller
+// resources. File inputs always derive their base from the selected file.
+func nativeFilesystemAccess(source Source, context *FilesystemContext) (*filesystemAccess, string, string, *Diagnostic) {
+	var namespace *nativeNamespace
+	rootPath := "."
+	basePath := "."
+	mainPath := ""
+	if source.kind == sourceFilesystem {
+		namespace = source.namespace
+		mainPath = source.path
+		basePath = path.Dir(source.path)
+		if context != nil && context.Root != "" {
+			rootPath = context.Root
+		}
+	} else if context != nil && context.FS != nil {
+		namespace = context.FS.namespace
+		rootPath, basePath = context.Root, context.BaseDir
+	}
+	if namespace == nil || nilFilesystem(namespace.filesystem) || !fs.ValidPath(rootPath) || !fs.ValidPath(basePath) {
+		value := optionDiagnostic(CodeOptionsInvalid, "filesystem context is invalid")
+		return nil, "", "", &value
+	}
+	access := newNativeAccess(namespace.filesystem, rootPath, namespace.token)
+	access.label = namespace.label
+	if !access.contains(basePath) || mainPath != "" && !access.contains(mainPath) {
+		value := importDiagnostic(CodeImportOutsideRoot, "selected source and import base must stay inside the configuration root", nil)
+		return nil, "", "", &value
+	}
+	return access, basePath, mainPath, nil
+}
+
+func validateNativeDirectories(access *filesystemAccess, basePath, mainPath string) *Diagnostic {
+	if !access.contains(basePath) || mainPath != "" && !access.contains(mainPath) {
+		value := importDiagnostic(CodeImportOutsideRoot, "selected source and import base must stay inside the configuration root", nil)
+		return &value
+	}
+	for _, directory := range []string{access.rootPath, basePath} {
+		info, err := access.stat(directory)
+		if err != nil || !info.IsDir() {
+			value := importDiagnostic(CodeImportInvalid, "configuration root and import base must be directories", nil)
+			return &value
+		}
+	}
+	return nil
+}
+
 func importFilesystem(source Source, context *FilesystemContext, main parsedSource) (rootPath, basePath, mainPath string, diagnostic *Diagnostic) {
 	if main.source == nil || main.document == nil {
 		value := importDiagnostic(CodeImportInvalid, "imports require a selected configuration source", nil)

internal/engine/filesystem_access.go

@@ -0,0 +1,185 @@
+package engine
+
+import (
+	"errors"
+	"io"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+)
+
+// namespaceToken identifies one caller-selected filesystem without comparing
+// arbitrary fs.FS values. A token can be shared by copies of a source wrapper.
+type namespaceToken struct{ marker byte }
+
+type nativeNamespace struct {
+	filesystem fs.FS
+	label      string
+	token      *namespaceToken
+}
+
+func newNativeNamespace(label string, filesystem fs.FS) *nativeNamespace {
+	return &nativeNamespace{filesystem: filesystem, label: label, token: &namespaceToken{}}
+}
+
+type resourceIdentity struct {
+	namespace  *namespaceToken
+	path       string
+	occurrence *namespaceToken
+}
+
+// filesystemAccess is a load-scoped view of either a contained host root or a
+// borrowed native filesystem. Names are canonical host paths or fs.FS paths.
+type filesystemAccess struct {
+	rootPath string
+	root     *os.Root
+	native   fs.FS
+	token    *namespaceToken
+	label    string
+}
+
+func openHostAccess(rootPath string) (*filesystemAccess, error) {
+	root, err := os.OpenRoot(rootPath)
+	if err != nil {
+		return nil, err
+	}
+	return &filesystemAccess{rootPath: rootPath, root: root, token: &namespaceToken{}}, nil
+}
+
+func newNativeAccess(filesystem fs.FS, root string, token *namespaceToken) *filesystemAccess {
+	return &filesystemAccess{rootPath: root, native: filesystem, token: token}
+}
+
+func (a *filesystemAccess) close() error {
+	if a.root != nil {
+		return a.root.Close()
+	}
+	return nil
+}
+
+func (a *filesystemAccess) identity(name string) resourceIdentity {
+	return resourceIdentity{namespace: a.token, path: name}
+}
+
+func (a *filesystemAccess) syntheticIdentity() resourceIdentity {
+	return resourceIdentity{namespace: a.token, occurrence: &namespaceToken{}}
+}
+
+func (a *filesystemAccess) join(base, name string) string {
+	if a.native != nil {
+		return path.Join(base, name)
+	}
+	return filepath.Join(base, filepath.FromSlash(path.Clean(name)))
+}
+
+func (a *filesystemAccess) dir(name string) string {
+	if a.native != nil {
+		return path.Dir(name)
+	}
+	return filepath.Dir(name)
+}
+
+func (a *filesystemAccess) contains(name string) bool {
+	if a.native == nil {
+		return containedPath(a.rootPath, filepath.Clean(name))
+	}
+	if !fs.ValidPath(name) || !fs.ValidPath(a.rootPath) {
+		return false
+	}
+	return a.rootPath == "." || name == a.rootPath || strings.HasPrefix(name, a.rootPath+"/")
+}
+
+func (a *filesystemAccess) canonical(name string) (string, error) {
+	if a.native == nil {
+		return canonicalExistingPath(name)
+	}
+	if !fs.ValidPath(name) {
+		return "", fs.ErrInvalid
+	}
+	_, err := fs.Stat(a.native, name)
+	if err != nil {
+		return "", err
+	}
+	return name, nil
+}
+
+func (a *filesystemAccess) relative(name string) (string, error) {
+	if a.native != nil {
+		if !a.contains(name) {
+			return "", fs.ErrInvalid
+		}
+		return name, nil
+	}
+	relative, err := filepath.Rel(a.rootPath, name)
+	if err != nil || filepath.IsAbs(relative) || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
+		return "", fs.ErrInvalid
+	}
+	return relative, nil
+}
+
+func (a *filesystemAccess) stat(name string) (fs.FileInfo, error) {
+	if a.native != nil {
+		return fs.Stat(a.native, name)
+	}
+	relative, err := a.relative(name)
+	if err != nil {
+		return nil, err
+	}
+	return a.root.Stat(relative)
+}
+
+func (a *filesystemAccess) open(name string) (fs.File, error) {
+	if a.native != nil {
+		return a.native.Open(name)
+	}
+	relative, err := a.relative(name)
+	if err != nil {
+		return nil, err
+	}
+	return a.root.Open(relative)
+}
+
+func (a *filesystemAccess) readDirectory(name string, visit func(fs.DirEntry) bool) error {
+	file, err := a.open(name)
+	if err != nil {
+		return err
+	}
+	if reader, ok := file.(fs.ReadDirFile); ok {
+		for {
+			entries, readErr := reader.ReadDir(globReadBatch)
+			for _, entry := range entries {
+				if !visit(entry) {
+					return file.Close()
+				}
+			}
+			if errors.Is(readErr, io.EOF) {
+				return file.Close()
+			}
+			if readErr != nil {
+				_ = file.Close()
+				return readErr
+			}
+			if len(entries) == 0 {
+				_ = file.Close()
+				return io.ErrNoProgress
+			}
+		}
+	}
+	if err := file.Close(); err != nil {
+		return err
+	}
+	// fs.ReadDir provides the standard fallback for implementations whose opened
+	// files do not implement fs.ReadDirFile. It may allocate all entries itself.
+	entries, err := fs.ReadDir(a.native, name)
+	if err != nil {
+		return err
+	}
+	for _, entry := range entries {
+		if !visit(entry) {
+			break
+		}
+	}
+	return nil
+}

internal/engine/glob.go

@@ -2,11 +2,8 @@ package engine
 
 import (
 	"errors"
-	"io"
 	"io/fs"
-	"os"
 	"path"
-	"path/filepath"
 	"slices"
 	"strings"
 )
@@ -42,29 +39,29 @@ func (s *importTraversal) expandImportGlob(basePath string, declaration importDe
 				names = []string{literal}
 			}
 			for _, name := range names {
-				absolute := filepath.Join(candidate.absolute, filepath.FromSlash(name))
+				absolute := s.access.join(candidate.absolute, name)
 				relative := path.Join(candidate.relative, name)
-				if !containedPath(s.rootPath, filepath.Clean(absolute)) {
+				if !s.access.contains(absolute) {
 					return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob traversal is outside the configuration root", &location)}
 				}
-				canonical, err := canonicalExistingPath(absolute)
+				canonical, err := s.access.canonical(absolute)
 				if err != nil {
 					if errors.Is(err, fs.ErrNotExist) && !patterned {
 						continue
 					}
 					return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob match could not be canonicalized", location)}
 				}
-				if !containedPath(s.rootPath, canonical) {
+				if !s.access.contains(canonical) {
 					return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob traversal resolves outside the configuration root", &location)}
 				}
 				if last {
 					next = append(next, globCandidate{absolute: canonical, relative: relative})
-					if len(s.sources)+len(next) > s.limits.sources {
+					if s.accounting.sources+len(next) > s.limits.sources {
 						return nil, []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "glob matches exceed the remaining source-count limit", &location)}
 					}
 					continue
 				}
-				info, err := os.Stat(canonical)
+				info, err := s.access.stat(canonical)
 				if err != nil {
 					return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob traversal target could not be inspected", location)}
 				}
@@ -88,56 +85,47 @@ func (s *importTraversal) expandImportGlob(basePath string, declaration importDe
 }
 
 func (s *importTraversal) readGlobMatches(directory, pattern string, location Location) ([]string, []Diagnostic) {
-	canonical, err := canonicalExistingPath(directory)
+	canonical, err := s.access.canonical(directory)
 	if err != nil {
 		if errors.Is(err, fs.ErrNotExist) {
 			return nil, nil
 		}
 		return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob directory could not be canonicalized", location)}
 	}
-	if !containedPath(s.rootPath, canonical) {
+	if !s.access.contains(canonical) {
 		return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob directory resolves outside the configuration root", &location)}
 	}
-	relative, err := filepath.Rel(s.rootPath, canonical)
-	if err != nil || filepath.IsAbs(relative) || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
+	if _, err := s.access.relative(canonical); err != nil {
 		return nil, []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "glob directory could not be addressed inside the configuration root", &location)}
 	}
-	directoryFile, err := s.root.Open(relative)
-	if err != nil {
-		if errors.Is(err, fs.ErrNotExist) {
-			return nil, nil
-		}
-		return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob directory could not be opened", location)}
-	}
-
 	var matches []string
-	for {
-		entries, readErr := directoryFile.ReadDir(globReadBatch)
-		for _, entry := range entries {
-			s.usedEntries++
-			if s.usedEntries > s.limits.entries {
-				_ = directoryFile.Close()
-				return nil, []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "glob traversal exceeds the directory-entry visit limit", &location)}
+	var problem *Diagnostic
+	err = s.access.readDirectory(canonical, func(entry fs.DirEntry) bool {
+		s.accounting.entries++
+		if s.accounting.entries > s.limits.entries {
+			value := importDiagnostic(CodeImportLimitExceeded, "glob traversal exceeds the directory-entry visit limit", &location)
+			problem = &value
+			return false
 		}
 		matched, matchErr := path.Match(pattern, entry.Name())
 		if matchErr != nil {
-				_ = directoryFile.Close()
-				return nil, []Diagnostic{importDiagnostic(CodeImportInvalid, "import glob pattern is invalid", &location)}
+			value := importDiagnostic(CodeImportInvalid, "import glob pattern is invalid", &location)
+			problem = &value
+			return false
 		}
 		if matched {
 			matches = append(matches, entry.Name())
 		}
+		return true
+	})
+	if problem != nil {
+		return nil, []Diagnostic{*problem}
 	}
-		if errors.Is(readErr, io.EOF) {
-			break
+	if errors.Is(err, fs.ErrNotExist) {
+		return nil, nil
 	}
-		if readErr != nil {
-			_ = directoryFile.Close()
-			return nil, []Diagnostic{globFilesystemDiagnostic(readErr, "glob directory could not be read", location)}
-		}
-	}
-	if err := directoryFile.Close(); err != nil {
-		return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob directory could not be closed", location)}
+	if err != nil {
+		return nil, []Diagnostic{globFilesystemDiagnostic(err, "glob directory could not be traversed", location)}
 	}
 	slices.Sort(matches)
 	return matches, nil

internal/engine/history.go

@@ -6,6 +6,40 @@ import (
 
 const historyNodeLimit = 4_000_000
 
+type retainedNodeCounter struct {
+	seen  map[*document.Node]bool
+	count int
+	limit int
+}
+
+func newRetainedNodeCounter(limit int) *retainedNodeCounter {
+	return &retainedNodeCounter{seen: make(map[*document.Node]bool), limit: limit}
+}
+
+func (c *retainedNodeCounter) add(root *document.Node) bool {
+	if c == nil {
+		return true
+	}
+	stack := []*document.Node{root}
+	for len(stack) != 0 {
+		node := stack[len(stack)-1]
+		stack = stack[:len(stack)-1]
+		if node == nil || c.seen[node] {
+			continue
+		}
+		c.seen[node] = true
+		c.count++
+		if c.limit >= 0 && c.count > c.limit {
+			return false
+		}
+		for _, entry := range node.Mapping {
+			stack = append(stack, entry.Value)
+		}
+		stack = append(stack, node.Sequence...)
+	}
+	return true
+}
+
 // History applications borrow immutable policy-validated input trees. Metadata
 // records answer presence and origin queries; replay reconstructs value changes.
 // Replay reconciles with canonical decoding before domain resolution so both
@@ -31,7 +65,7 @@ func cloneHistoryApplications(values []historyApplication) []historyApplication
 	return cloned
 }
 
-func applicationOperationPath(application historyApplication, query Path) (Path, bool) {
+func applicationOperationPath(application historyApplication, query Path, lookup *documentLookup) (Path, bool) {
 	if !application.path.valid() || application.value == nil {
 		return Path{}, false
 	}
@@ -46,7 +80,7 @@ func applicationOperationPath(application historyApplication, query Path) (Path,
 	}
 	for length := 1; length <= len(query.segments); length++ {
 		prefix := Path{segments: clonePathSegments(query.segments[:length], 0)}
-		node, present := documentAtPath(application.value, prefix)
+		node, present := documentAtPathWithLookup(application.value, prefix, lookup)
 		if !present {
 			return Path{}, false
 		}
@@ -57,7 +91,7 @@ func applicationOperationPath(application historyApplication, query Path) (Path,
 	return Path{}, false
 }
 
-func historyApplicationOrigin(application historyApplication, operationPath Path) Origin {
+func historyApplicationOrigin(application historyApplication, operationPath Path, lookup *documentLookup) Origin {
 	if application.origin.Layer != "" {
 		return cloneOrigin(application.origin)
 	}
@@ -66,7 +100,7 @@ func historyApplicationOrigin(application historyApplication, operationPath Path
 	if !ok {
 		return origin
 	}
-	if node, present := documentAtPath(application.value, relative); present {
+	if node, present := documentAtPathWithLookup(application.value, relative, lookup); present {
 		origin.Locations = []Location{publicLocation(node.Location)}
 	}
 	return origin
@@ -83,15 +117,15 @@ func pathsIntersect(left, right Path) bool {
 	return pathContains(left, right) || pathContains(right, left)
 }
 
-func applyHistoryApplication(root **document.Node, application historyApplication) {
+func applyHistoryApplication(root **document.Node, application historyApplication, index *mappingIndex) {
 	if application.path.String() == "" {
-		mergeHistoryDocument(root, application.value)
+		mergeHistoryDocument(root, application.value, index)
 		return
 	}
-	mergeHistoryAtPath(root, application.path, application.value)
+	mergeHistoryAtPath(root, application.path, application.value, index)
 }
 
-func mergeHistoryDocument(target **document.Node, incoming *document.Node) {
+func mergeHistoryDocument(target **document.Node, incoming *document.Node, index *mappingIndex) {
 	if incoming == nil {
 		return
 	}
@@ -100,39 +134,39 @@ func mergeHistoryDocument(target **document.Node, incoming *document.Node) {
 		return
 	}
 	for _, entry := range incoming.Mapping {
-		position := documentEntryIndex((*target).Mapping, entry.Key)
+		position := index.find(*target, entry.Key)
 		if position < 0 {
 			cloned := entry
 			cloned.Value = document.Clone(entry.Value)
-			(*target).Mapping = append((*target).Mapping, cloned)
+			index.append(*target, cloned)
 			continue
 		}
-		mergeHistoryDocument(&(*target).Mapping[position].Value, entry.Value)
+		mergeHistoryDocument(&(*target).Mapping[position].Value, entry.Value, index)
 	}
 }
 
-func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node) {
+func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node, index *mappingIndex) {
 	if root == nil || *root == nil || len(path.segments) == 0 {
-		mergeHistoryDocument(root, incoming)
+		mergeHistoryDocument(root, incoming, index)
 		return
 	}
 	current := *root
-	for index, segment := range path.segments {
+	for segmentIndex, segment := range path.segments {
 		if current.Kind != document.Mapping || segment.kind == pathSegmentIndex {
 			return
 		}
-		position := documentEntryIndex(current.Mapping, segment.key)
-		if index == len(path.segments)-1 {
+		position := index.find(current, segment.key)
+		if segmentIndex == len(path.segments)-1 {
 			if position < 0 {
-				current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, Value: document.Clone(incoming)})
+				index.append(current, document.Entry{Key: segment.key, Value: document.Clone(incoming)})
 			} else {
-				mergeHistoryDocument(&current.Mapping[position].Value, incoming)
+				mergeHistoryDocument(&current.Mapping[position].Value, incoming, index)
 			}
 			return
 		}
 		if position < 0 {
 			child := &document.Node{Kind: document.Mapping}
-			current.Mapping = append(current.Mapping, document.Entry{Key: segment.key, Value: child})
+			index.append(current, document.Entry{Key: segment.key, Value: child})
 			current = child
 		} else {
 			current = current.Mapping[position].Value
@@ -145,28 +179,50 @@ func mergeHistoryAtPath(root **document.Node, path Path, incoming *document.Node
 // intent into that event rather than replay a child assignment a second time.
 func coalescedResolutionSnapshots(snapshots []resolutionSnapshot) []resolutionSnapshot {
 	groups := make([]resolutionSnapshot, 0, len(snapshots))
+	reasons := make([]map[string]struct{}, 0, len(snapshots))
+	dependencies := make([]map[string]struct{}, 0, len(snapshots))
+	var coverage pathPrefixIndex
 	for _, snapshot := range snapshots {
-		merged := false
-		for i := range groups {
-			if !pathContains(groups[i].change.Path, snapshot.change.Path) {
-				continue
+		pointer := snapshot.change.Path.String()
+		groupIndex := -1
+		coverage.visitAncestors(pointer, func(id int) {
+			if groupIndex < 0 || id < groupIndex {
+				groupIndex = id
 			}
+		})
+		if groupIndex >= 0 {
+			group := &groups[groupIndex]
 			for _, reason := range snapshot.change.Reasons {
-				groups[i].change.Reasons = appendUniqueString(groups[i].change.Reasons, reason)
+				if _, found := reasons[groupIndex][reason]; !found {
+					reasons[groupIndex][reason] = struct{}{}
+					group.change.Reasons = append(group.change.Reasons, reason)
+				}
 			}
 			for _, dependency := range snapshot.change.Dependencies {
-				groups[i].change.Dependencies = appendUniquePath(groups[i].change.Dependencies, dependency)
+				dependencyPointer := dependency.String()
+				if _, found := dependencies[groupIndex][dependencyPointer]; !found {
+					dependencies[groupIndex][dependencyPointer] = struct{}{}
+					group.change.Dependencies = append(group.change.Dependencies, dependency.clone())
+				}
 			}
 			if snapshot.change.Origin.Operation == ChangeNormalization {
-				groups[i].change.Origin.Operation = ChangeNormalization
+				group.change.Origin.Operation = ChangeNormalization
 			}
-			merged = true
-			break
+			continue
 		}
-		if !merged {
 		snapshot.change = cloneChange(snapshot.change)
 		groups = append(groups, snapshot)
+		reasonSet := make(map[string]struct{}, len(snapshot.change.Reasons))
+		for _, reason := range snapshot.change.Reasons {
+			reasonSet[reason] = struct{}{}
 		}
+		reasons = append(reasons, reasonSet)
+		dependencySet := make(map[string]struct{}, len(snapshot.change.Dependencies))
+		for _, dependency := range snapshot.change.Dependencies {
+			dependencySet[dependency.String()] = struct{}{}
+		}
+		dependencies = append(dependencies, dependencySet)
+		coverage.insert(pointer, len(groups)-1)
 	}
 	return groups
 }
@@ -230,11 +286,11 @@ func replaceHistoryPath(root **document.Node, path Path, replacement *document.N
 	}
 }
 
-func historyNodesWithinLimit(baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot) bool {
-	return historyNodesWithin(historyNodeLimit, baseline, input, final, applications, resolutions)
+func historyNodesWithinLimit(baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot, inherited ...*inheritanceScope) bool {
+	return historyNodesWithin(historyNodeLimit, baseline, input, final, applications, resolutions, inherited...)
 }
 
-func historyNodesWithin(limit int, baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot) bool {
+func historyNodesWithin(limit int, baseline, input, final *document.Node, applications []historyApplication, resolutions []resolutionSnapshot, inherited ...*inheritanceScope) bool {
 	seen := make(map[*document.Node]bool)
 	count := 0
 	var visit func(*document.Node) bool
@@ -272,5 +328,23 @@ func historyNodesWithin(limit int, baseline, input, final *document.Node, applic
 			return false
 		}
 	}
+	for _, expansion := range inherited {
+		if expansion == nil {
+			continue
+		}
+		if !visit(expansion.root) {
+			return false
+		}
+		for _, operation := range expansion.operations {
+			if !visit(operation.parent) || !visit(operation.effective) {
+				return false
+			}
+		}
+		for _, replacement := range expansion.replacements {
+			if !visit(replacement.before) || !visit(replacement.after) {
+				return false
+			}
+		}
+	}
 	return true
 }

internal/engine/imports.go

@@ -4,9 +4,6 @@ import (
 	"bytes"
 	"errors"
 	"io/fs"
-	"os"
-	"path"
-	"path/filepath"
 	"strconv"
 	"strings"
 
@@ -40,47 +37,96 @@ type importRoute struct {
 	locations []Location
 }
 
+type importAccounting struct {
+	sources int
+	bytes   int64
+	nodes   int
+	entries int
+}
+
 type importTraversal struct {
-	rootPath string
-	root     *os.Root
+	access     *filesystemAccess
+	accounting *importAccounting
 	limits     importLimits
 	rule       *VersionRule
 
 	documents []*document.Node
 	sources   []SourceInfo
-	active      map[string]int
-	visited     map[string]importRoute
+	active    map[resourceIdentity]int
+	visited   map[resourceIdentity]importRoute
 	stack     []importRoute
-	usedBytes   int64
-	usedNodes   int
-	usedEntries int
 }
 
 func loadImportedDocuments(source Source, context *FilesystemContext, main parsedSource, rule *VersionRule, limits importLimits) (*composedDocument, []SourceInfo, []Diagnostic) {
+	return loadImportedDocumentsTracked(source, context, main, rule, limits, &importAccounting{}, nil, nil)
+}
+
+func loadImportedDocumentsTracked(source Source, context *FilesystemContext, main parsedSource, rule *VersionRule, limits importLimits, accounting *importAccounting, visited map[resourceIdentity]importRoute, hostToken *namespaceToken) (*composedDocument, []SourceInfo, []Diagnostic) {
+	if nativeContext(source, context) {
+		access, basePath, mainPath, diagnostic := nativeFilesystemAccess(source, context)
+		if diagnostic != nil {
+			return nil, sourceInfos(main.source), []Diagnostic{*diagnostic}
+		}
+		return loadImportedWithAccessTracked(access, basePath, mainPath, main, rule, limits, accounting, visited)
+	}
 	rootPath, basePath, mainPath, diagnostic := importFilesystem(source, context, main)
 	if diagnostic != nil {
 		return nil, sourceInfos(main.source), []Diagnostic{*diagnostic}
 	}
-	root, err := os.OpenRoot(rootPath)
+	access, err := openHostAccess(rootPath)
 	if err != nil {
 		diagnostic := globFilesystemDiagnostic(err, "configuration root could not be opened", Location{})
 		diagnostic.Primary = nil
 		return nil, sourceInfos(main.source), []Diagnostic{diagnostic}
 	}
+	if hostToken != nil {
+		access.token = hostToken
+	}
+	return loadImportedWithAccessTracked(access, basePath, mainPath, main, rule, limits, accounting, visited)
+}
 
+func loadImportedWithAccess(access *filesystemAccess, basePath, mainPath string, main parsedSource, rule *VersionRule, limits importLimits, accounting *importAccounting) (*composedDocument, []SourceInfo, []Diagnostic) {
+	return loadImportedWithAccessTracked(access, basePath, mainPath, main, rule, limits, accounting, nil)
+}
+
+func loadImportedWithAccessTracked(access *filesystemAccess, basePath, mainPath string, main parsedSource, rule *VersionRule, limits importLimits, accounting *importAccounting, visited map[resourceIdentity]importRoute) (*composedDocument, []SourceInfo, []Diagnostic) {
+	if !access.contains(basePath) || (mainPath != "" && !access.contains(mainPath)) {
+		_ = access.close()
+		return nil, sourceInfos(main.source), []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "selected source and import base must stay inside the configuration root", nil)}
+	}
+	for _, directory := range []string{access.rootPath, basePath} {
+		info, err := access.stat(directory)
+		if err != nil || !info.IsDir() {
+			_ = access.close()
+			return nil, sourceInfos(main.source), []Diagnostic{importDiagnostic(CodeImportInvalid, "configuration root and import base must be directories", nil)}
+		}
+	}
 	mainInfo := cloneSourceInfo(*main.source)
 	if mainPath != "" {
 		mainInfo.Path = mainPath
 	}
+	mainKey := access.identity(mainPath)
+	if mainPath == "" {
+		mainKey = access.syntheticIdentity()
+	}
+	if previous, repeated := visited[mainKey]; repeated {
+		_ = access.close()
+		return nil, sourceInfos(main.source), []Diagnostic{importRepeatedDiagnostic(Location{SourceID: main.source.ID}, previous, importRoute{})}
+	}
+	accounting.sources++
+	accounting.bytes += main.bytes
+	accounting.nodes += countInputNodes(main.document)
 	state := importTraversal{
-		rootPath: rootPath, root: root, limits: limits, rule: rule,
+		access: access, accounting: accounting, limits: limits, rule: rule,
 		documents: make([]*document.Node, 0, 1),
 		sources:   []SourceInfo{mainInfo},
-		active:    make(map[string]int), visited: make(map[string]importRoute),
-		usedBytes: main.bytes, usedNodes: countInputNodes(main.document),
+		active:    make(map[resourceIdentity]int), visited: visited,
+	}
+	if state.visited == nil {
+		state.visited = make(map[resourceIdentity]importRoute)
 	}
 	if diagnostic := state.checkInitialLimits(); diagnostic != nil {
-		_ = root.Close()
+		_ = access.close()
 		return nil, cloneSourceInfos(state.sources), []Diagnostic{*diagnostic}
 	}
 
@@ -89,8 +135,8 @@ func loadImportedDocuments(source Source, context *FilesystemContext, main parse
 		mainIdentity = "<selected-source>"
 	}
 	mainRoute := importRoute{canonical: mainIdentity}
-	state.active[mainIdentity] = 0
-	state.visited[mainIdentity] = mainRoute
+	state.active[mainKey] = 0
+	state.visited[mainKey] = mainRoute
 	state.stack = append(state.stack, mainRoute)
 	body, declarations, diagnostics := splitImportDirectives(main.document)
 	if len(diagnostics) == 0 {
@@ -98,7 +144,7 @@ func loadImportedDocuments(source Source, context *FilesystemContext, main parse
 		diagnostics = state.walkDeclarations(basePath, declarations, 0)
 	}
 
-	closeErr := root.Close()
+	closeErr := access.close()
 	if len(diagnostics) != 0 {
 		return nil, cloneSourceInfos(state.sources), diagnostics
 	}
@@ -107,22 +153,22 @@ func loadImportedDocuments(source Source, context *FilesystemContext, main parse
 	}
 	composed, diagnostics := composeDocuments(state.documents...)
 	if composed != nil {
-		composed.inputBytes = state.usedBytes
-		composed.inputNodes = state.usedNodes
+		composed.inputBytes = accounting.bytes
+		composed.inputNodes = accounting.nodes
 	}
 	return composed, cloneSourceInfos(state.sources), diagnostics
 }
 
 func (s *importTraversal) checkInitialLimits() *Diagnostic {
-	if s.limits.sources < 1 {
+	if s.accounting.sources > s.limits.sources {
 		value := importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the source-count limit", nil)
 		return &value
 	}
-	if s.usedBytes > s.limits.bytes {
+	if s.accounting.bytes > s.limits.bytes {
 		value := importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate byte limit", nil)
 		return &value
 	}
-	if s.usedNodes > s.limits.nodes {
+	if s.accounting.nodes > s.limits.nodes {
 		value := importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", nil)
 		return &value
 	}
@@ -169,11 +215,11 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 		return []Diagnostic{importDiagnostic(CodeImportInvalid, "import path must be a nonempty slash-relative path without a network scheme", &location)}
 	}
 
-	candidate := filepath.Join(basePath, filepath.FromSlash(path.Clean(declaration.path)))
-	if !containedPath(s.rootPath, filepath.Clean(candidate)) {
+	candidate := s.access.join(basePath, declaration.path)
+	if !s.access.contains(candidate) {
 		return []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "import target is outside the configuration root", &location)}
 	}
-	canonical, err := canonicalExistingPath(candidate)
+	canonical, err := s.access.canonical(candidate)
 	if err != nil {
 		if declaration.optional && errors.Is(err, fs.ErrNotExist) {
 			return nil
@@ -187,30 +233,39 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 		}
 		return []Diagnostic{importDiagnostic(code, message, &location)}
 	}
-	if !containedPath(s.rootPath, canonical) {
+	if !s.access.contains(canonical) {
 		return []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "import target resolves outside the configuration root", &location)}
 	}
 
 	currentRoute := s.currentRoute(location)
-	if activeIndex, ok := s.active[canonical]; ok {
+	identity := s.access.identity(canonical)
+	if activeIndex, ok := s.active[identity]; ok {
 		cycle := append([]importRoute(nil), s.stack[activeIndex:]...)
 		return []Diagnostic{importCycleDiagnostic(canonical, location, cycle)}
 	}
-	if previous, ok := s.visited[canonical]; ok {
+	if previous, ok := s.visited[identity]; ok {
 		return []Diagnostic{importRepeatedDiagnostic(location, previous, currentRoute)}
 	}
-	if len(s.sources)+1 > s.limits.sources {
+	if s.accounting.sources+1 > s.limits.sources {
 		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the source-count limit", &location)}
 	}
 
-	relative, err := filepath.Rel(s.rootPath, canonical)
-	if err != nil || filepath.IsAbs(relative) || relative == ".." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) {
+	if _, err := s.access.relative(canonical); err != nil {
 		return []Diagnostic{importDiagnostic(CodeImportOutsideRoot, "import target could not be addressed inside the configuration root", &location)}
 	}
-	sourceID := "source-" + strconv.Itoa(len(s.sources)+1)
-	info := SourceInfo{ID: sourceID, Label: declaration.path, Path: canonical, Layer: LayerBase, ImportedFrom: &location}
+	sourceID := "source-" + strconv.Itoa(s.accounting.sources+1)
+	kind := SourceKindFile
+	label := declaration.path
+	if s.access.native != nil {
+		kind = SourceKindFilesystem
+		label = s.access.label + "/" + canonical
+	}
+	info := SourceInfo{ID: sourceID, Label: label, Path: canonical, Layer: LayerBase, Kind: kind, ImportedFrom: &location}
+	if kind == SourceKindFilesystem {
+		info.namespace = s.access.token
+	}
 	s.sources = append(s.sources, info)
-	data, skipped, diagnostic := s.readImport(relative, sourceID, declaration.optional)
+	data, skipped, diagnostic := s.readImport(canonical, sourceID, declaration.optional)
 	if diagnostic != nil {
 		return []Diagnostic{*diagnostic}
 	}
@@ -218,6 +273,7 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 		s.sources = s.sources[:len(s.sources)-1]
 		return nil
 	}
+	s.accounting.sources++
 
 	root, issues := yamladapter.Parse(bytes.NewReader(data), sourceID, yamladapter.Limits{
 		Depth: maxDocumentDepth, Nodes: maxDocumentNodes,
@@ -226,10 +282,10 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 		return parserDiagnostics(sourceID, issues)
 	}
 	nodeCount := countInputNodes(root)
-	if s.usedNodes+nodeCount > s.limits.nodes {
+	if s.accounting.nodes+nodeCount > s.limits.nodes {
 		return []Diagnostic{importDiagnostic(CodeImportLimitExceeded, "configuration exceeds the aggregate node limit", &location)}
 	}
-	s.usedNodes += nodeCount
+	s.accounting.nodes += nodeCount
 
 	body, declarations, diagnostics := splitImportDirectives(root)
 	if len(diagnostics) != 0 {
@@ -245,22 +301,22 @@ func (s *importTraversal) visitPath(basePath string, declaration importDeclarati
 
 	route := importRoute{canonical: canonical, locations: currentRoute.locations}
 	s.documents = append(s.documents, body)
-	s.active[canonical] = len(s.stack)
-	s.visited[canonical] = route
+	s.active[identity] = len(s.stack)
+	s.visited[identity] = route
 	s.stack = append(s.stack, route)
-	diagnostics = s.walkDeclarations(filepath.Dir(canonical), declarations, depth)
-	delete(s.active, canonical)
+	diagnostics = s.walkDeclarations(s.access.dir(canonical), declarations, depth)
+	delete(s.active, identity)
 	s.stack = s.stack[:len(s.stack)-1]
 	return diagnostics
 }
 
-func (s *importTraversal) readImport(relative, sourceID string, optional bool) ([]byte, bool, *Diagnostic) {
-	remaining := s.limits.bytes - s.usedBytes
+func (s *importTraversal) readImport(name, sourceID string, optional bool) ([]byte, bool, *Diagnostic) {
+	remaining := s.limits.bytes - s.accounting.bytes
 	limit := min(int64(maxSourceBytes), remaining)
 	if limit < 0 {
 		limit = 0
 	}
-	data, failure := readRegularFile(relative, s.root.Stat, s.root.Open, limit)
+	data, failure := readRegularFile(name, s.access.stat, s.access.open, limit)
 	if failure != nil {
 		if optional && failure.missingTarget {
 			return nil, true, nil
@@ -268,7 +324,7 @@ func (s *importTraversal) readImport(relative, sourceID string, optional bool) (
 		value := sourceReadDiagnostic(failure, sourceID, CodeImportLimitExceeded, "configuration exceeds a source or aggregate byte limit")
 		return nil, false, &value
 	}
-	s.usedBytes += int64(len(data))
+	s.accounting.bytes += int64(len(data))
 	return data, false, nil
 }
 

internal/engine/inheritance.go

@@ -0,0 +1,313 @@
+package engine
+
+import (
+	"slices"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+const inheritanceKey = "extends"
+
+// These private records keep structural evidence separate from acquired source
+// records. Later inspection can project a transfer onto individual descendants.
+type inheritanceTransfer struct {
+	from      Path
+	to        Path
+	reference Path
+	locations []Location
+}
+
+type inheritanceControl struct {
+	path      Path
+	locations []Location
+	cancelled bool
+}
+
+type inheritanceScope struct {
+	root         *document.Node
+	origins      map[string]Origin
+	transfers    []inheritanceTransfer
+	controls     []inheritanceControl
+	replacements []inheritanceReplacement
+	operations   []inheritanceOperation
+}
+
+type inheritanceOperation struct {
+	control          *inheritanceControl
+	transfer         *inheritanceTransfer
+	controlOrigin    Origin
+	parent           *document.Node
+	effective        *document.Node
+	parentOrigins    map[string]Origin
+	effectiveOrigins map[string]Origin
+}
+
+// A replacement retains the displaced source/derivation record while the
+// effective origin belongs to the child. Value trees are borrowed immutable
+// inputs or detached templates and are budgeted with retained history later.
+type inheritanceReplacement struct {
+	path      Path
+	before    *document.Node
+	after     *document.Node
+	previous  []displacedDefinition
+	effective Origin
+}
+
+// A link represents one shared relationship edge. Each contributor retains
+// its original semantic path independently of source line/column coordinates.
+type inheritanceTrace struct {
+	previous *inheritanceTrace
+	transfer *inheritanceTransfer
+}
+
+type inheritanceContributor struct {
+	source   Path
+	location Location
+	trace    *inheritanceTrace
+}
+
+type siblingFrame struct {
+	name      string
+	parent    string
+	hasParent bool
+	entered   bool
+}
+
+// expandSiblingDefinitions resolves only the mappings directly in scope.
+// Nested instructions remain in the returned templates for destination-local
+// resolution. This private entry point does not participate in public loading.
+func expandSiblingDefinitions(scope *document.Node, scopePath Path) (*inheritanceScope, []Diagnostic) {
+	return expandSiblingDefinitionsWithLimits(scope, scopePath, defaultProjectionLimits())
+}
+
+func expandSiblingDefinitionsWithLimits(scope *document.Node, scopePath Path, limits projectionLimits) (*inheritanceScope, []Diagnostic) {
+	return expandSiblingDefinitionsWithOrigins(scope, scopePath, limits, nil)
+}
+
+func expandSiblingDefinitionsWithOrigins(scope *document.Node, scopePath Path, limits projectionLimits, inputOrigins map[string]Origin, budget ...*retainedNodeCounter) (*inheritanceScope, []Diagnostic) {
+	if scope == nil || scope.Kind != document.Mapping {
+		return nil, []Diagnostic{{
+			Code: CodeInheritanceInvalid, Severity: SeverityError, Path: scopePath,
+			Message: "inheritance scope must be a mapping",
+		}}
+	}
+
+	byName := make(map[string]document.Entry, len(scope.Mapping))
+	names := make([]string, 0, len(scope.Mapping))
+	for _, entry := range scope.Mapping {
+		byName[entry.Key] = entry
+		names = append(names, entry.Key)
+	}
+	slices.Sort(names)
+
+	resolved := make(map[string]*document.Node, len(byName))
+	resolvedOrigins := make(map[string]map[string]Origin, len(byName))
+	state := make(map[string]uint8, len(byName))
+	result := &inheritanceScope{}
+	counter := newRetainedNodeCounter(historyNodeLimit)
+	if len(budget) != 0 {
+		counter = budget[0]
+	}
+	retained := 1 // scope mapping node
+	for _, entry := range scope.Mapping {
+		if entry.Value.Kind == document.Mapping {
+			continue
+		}
+		remaining := limits.nodes - retained - 1
+		if limits.nodes >= 0 && remaining < 0 {
+			return nil, []Diagnostic{inheritanceBudgetDiagnostic(scopePath.Key(entry.Key), entry.Value.Location)}
+		}
+		count, within := inheritanceLowerBoundCount(entry.Value, remaining)
+		if !within {
+			return nil, []Diagnostic{inheritanceBudgetDiagnostic(scopePath.Key(entry.Key), entry.Value.Location)}
+		}
+		retained += 1 + count // key and value subtree
+	}
+	for _, name := range names {
+		if byName[name].Value.Kind != document.Mapping || state[name] == 2 {
+			continue
+		}
+		stack := []siblingFrame{{name: name}}
+		for len(stack) != 0 {
+			frame := &stack[len(stack)-1]
+			entry := byName[frame.name]
+			path := scopePath.Key(frame.name)
+			if !frame.entered {
+				state[frame.name] = 1
+				frame.entered = true
+				reference, found := inheritanceDirective(entry.Value)
+				if found {
+					referencePath := path.Key(inheritanceKey)
+					location := publicLocation(reference.Value.Location)
+					result.controls = append(result.controls, inheritanceControl{
+						path: referencePath, locations: []Location{location},
+						cancelled: reference.Value.Kind == document.Null,
+					})
+					switch reference.Value.Kind {
+					case document.Null:
+					case document.String:
+						frame.parent, frame.hasParent = reference.Value.Text, true
+						parent, exists := byName[frame.parent]
+						if !exists {
+							return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceMissing, referencePath, location, nil,
+								"referenced sibling does not exist")}
+						}
+						if parent.Value.Kind != document.Mapping {
+							related := []Location{publicLocation(parent.Value.Location)}
+							return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceInvalid, referencePath, location, related,
+								"referenced sibling is not a mapping")}
+						}
+						if state[frame.parent] == 1 {
+							related := []Location{publicLocation(parent.Value.Location)}
+							return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceCycle, referencePath, location, related,
+								"sibling inheritance contains a cycle")}
+						}
+						if state[frame.parent] == 0 {
+							stack = append(stack, siblingFrame{name: frame.parent})
+							continue
+						}
+					default:
+						return nil, []Diagnostic{inheritanceDiagnostic(CodeInheritanceInvalid, referencePath, location, nil,
+							"inheritance reference must be a sibling name or null")}
+					}
+				}
+			}
+
+			payload := withoutInheritanceDirective(entry.Value)
+			operationStart, replacementStart := len(result.operations), len(result.replacements)
+			childOrigins := directInheritanceOrigins(payload, path, inputOrigins)
+			if reference, found := inheritanceDirective(entry.Value); found {
+				control := inheritanceControl{
+					path:      path.Key(inheritanceKey),
+					locations: []Location{publicLocation(reference.Value.Location)},
+					cancelled: reference.Value.Kind == document.Null,
+				}
+				controlOrigins := directInheritanceOrigins(reference.Value, control.path, inputOrigins)
+				result.operations = append(result.operations, inheritanceOperation{
+					control: &control, controlOrigin: controlOrigins[control.path.String()],
+				})
+			}
+			if limits.nodes >= 0 {
+				remaining := limits.nodes - retained - 1 // the sibling key
+				within := false
+				if frame.hasParent {
+					within = inheritanceOverlayLowerBoundWithin(resolved[frame.parent], payload, remaining)
+				} else {
+					within = inheritanceLowerBoundWithin(payload, remaining)
+				}
+				if !within {
+					return nil, []Diagnostic{inheritanceBudgetDiagnostic(path, entry.Value.Location)}
+				}
+			}
+			if frame.hasParent {
+				parent := resolved[frame.parent]
+				merged := document.Clone(parent)
+				control := inheritanceDirectiveLocation(entry.Value)
+				transfer := inheritanceTransfer{
+					from: scopePath.Key(frame.parent), to: path,
+					reference: path.Key(inheritanceKey), locations: []Location{control},
+				}
+				mergedOrigins := transferredInheritanceOrigins(resolvedOrigins[frame.parent], transfer.from, path, transfer)
+				mergeInheritanceOrigin(mergedOrigins, path, childOrigins[path.String()])
+				mergeRecursiveMappings(merged, payload, path, new(mappingIndex), recursiveMergeHooks{
+					added: func(childPath Path, incoming *document.Node) {
+						copyInheritanceSubtree(mergedOrigins, childOrigins, incoming, childPath)
+					},
+					combined: func(childPath Path, _ *document.Node) {
+						mergeInheritanceOrigin(mergedOrigins, childPath, childOrigins[childPath.String()])
+					},
+					replaced: func(childPath Path, existing, incoming *document.Node) {
+						previous := displacedOrigins(existing, mergedOrigins, childPath)
+						result.replacements = append(result.replacements, inheritanceReplacement{
+							path: childPath.clone(), before: existing, after: incoming,
+							previous: previous, effective: cloneOrigin(childOrigins[childPath.String()]),
+						})
+						deleteOriginSubtree(mergedOrigins, childPath, existing)
+						copyInheritanceSubtree(mergedOrigins, childOrigins, incoming, childPath)
+					},
+				})
+				merged.Location = entry.Value.Location
+				resolved[frame.name] = merged
+				resolvedOrigins[frame.name] = mergedOrigins
+				result.transfers = append(result.transfers, transfer)
+				result.operations = append(result.operations, inheritanceOperation{
+					transfer: &transfer, parent: parent, effective: merged,
+					parentOrigins: resolvedOrigins[frame.parent], effectiveOrigins: mergedOrigins,
+				})
+			} else {
+				resolved[frame.name] = document.Clone(payload)
+				resolvedOrigins[frame.name] = childOrigins
+			}
+			count, _ := inheritanceLowerBoundCount(resolved[frame.name], limits.nodes)
+			retained += 1 + count
+			for _, operation := range result.operations[operationStart:] {
+				if !counter.add(operation.parent) || !counter.add(operation.effective) {
+					return nil, []Diagnostic{inheritanceHistoryBudgetDiagnostic(path, entry.Value.Location)}
+				}
+			}
+			for _, replacement := range result.replacements[replacementStart:] {
+				if !counter.add(replacement.before) || !counter.add(replacement.after) {
+					return nil, []Diagnostic{inheritanceHistoryBudgetDiagnostic(path, entry.Value.Location)}
+				}
+			}
+			state[frame.name] = 2
+			stack = stack[:len(stack)-1]
+		}
+	}
+
+	root := *scope
+	root.Mapping = make([]document.Entry, len(scope.Mapping))
+	rootOrigin := directInheritanceOrigins(&document.Node{Kind: document.Mapping, Location: scope.Location}, scopePath, inputOrigins)
+	result.origins = rootOrigin
+	for index, entry := range scope.Mapping {
+		root.Mapping[index] = entry
+		if resolved[entry.Key] != nil {
+			root.Mapping[index].Value = resolved[entry.Key]
+			for pointer, origin := range resolvedOrigins[entry.Key] {
+				result.origins[pointer] = origin
+			}
+		} else {
+			root.Mapping[index].Value = document.Clone(entry.Value)
+			for pointer, origin := range directInheritanceOrigins(entry.Value, scopePath.Key(entry.Key), inputOrigins) {
+				result.origins[pointer] = origin
+			}
+		}
+	}
+	result.root = &root
+	if !inheritanceLowerBoundWithin(result.root, limits.nodes) {
+		return nil, []Diagnostic{inheritanceBudgetDiagnostic(scopePath, scope.Location)}
+	}
+	return result, nil
+}
+
+func inheritanceDirective(node *document.Node) (document.Entry, bool) {
+	for _, entry := range node.Mapping {
+		if entry.Key == inheritanceKey {
+			return entry, true
+		}
+	}
+	return document.Entry{}, false
+}
+
+func inheritanceDirectiveLocation(node *document.Node) Location {
+	entry, _ := inheritanceDirective(node)
+	return publicLocation(entry.Value.Location)
+}
+
+func withoutInheritanceDirective(node *document.Node) *document.Node {
+	payload := *node
+	payload.Mapping = make([]document.Entry, 0, len(node.Mapping))
+	for _, entry := range node.Mapping {
+		if entry.Key != inheritanceKey {
+			payload.Mapping = append(payload.Mapping, entry)
+		}
+	}
+	return &payload
+}
+
+func inheritanceDiagnostic(code Code, path Path, primary Location, related []Location, message string) Diagnostic {
+	return Diagnostic{
+		Code: code, Severity: SeverityError, Path: path, Message: message,
+		Primary: &primary, Related: related,
+	}
+}

internal/engine/inheritance_bench_test.go

@@ -0,0 +1,110 @@
+package engine
+
+import (
+	"strconv"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func BenchmarkInheritanceExpansion(b *testing.B) {
+	for _, shape := range []string{"chain", "wide", "nested", "replacement"} {
+		for _, size := range []int{16, 32, 64} {
+			b.Run(shape+"/"+strconv.Itoa(size), func(b *testing.B) {
+				input := inheritanceBenchmarkInput(shape, size)
+				b.ReportAllocs()
+				b.ResetTimer()
+				for range b.N {
+					if _, diagnostics := expandInheritanceWithOrigins(input, nil, defaultProjectionLimits()); len(diagnostics) != 0 {
+						b.Fatal(diagnostics)
+					}
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkInheritanceInspectedLoad(b *testing.B) {
+	type entry struct {
+		Value string `yaml:"value"`
+	}
+	type configuration struct {
+		Entries map[string]entry `yaml:"entries"`
+	}
+	for _, size := range []int{16, 32, 64} {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			var input strings.Builder
+			input.WriteString("entries:\n  item-0: {value: base}\n")
+			for index := 1; index <= size; index++ {
+				input.WriteString("  item-")
+				input.WriteString(strconv.Itoa(index))
+				input.WriteString(": {extends: item-0}\n")
+			}
+			source := Bytes("inheritance-benchmark", []byte(input.String()))
+			query := Keys("entries", "item-"+strconv.Itoa(size), "value")
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				loaded, err := Load[configuration](source, testSecretBinding())
+				if err != nil {
+					b.Fatal(err)
+				}
+				if _, found := loaded.Inspection.Explain(query); !found {
+					b.Fatal("inherited value is not explainable")
+				}
+			}
+		})
+	}
+}
+
+// Each scope has constant-sized output and evidence. This distinguishes budget
+// accounting overhead from legitimate growth in expanded parent payloads.
+func BenchmarkInheritanceIndependentScopes(b *testing.B) {
+	for _, size := range []int{256, 512, 1024} {
+		b.Run(strconv.Itoa(size), func(b *testing.B) {
+			input := &document.Node{Kind: document.Mapping}
+			for index := 0; index < size; index++ {
+				input.Mapping = append(input.Mapping, document.Entry{
+					Key: strconv.Itoa(index), Value: inheritanceBenchmarkInput("wide", 1),
+				})
+			}
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				if _, diagnostics := expandInheritanceTree(input, defaultProjectionLimits()); len(diagnostics) != 0 {
+					b.Fatal(diagnostics)
+				}
+			}
+		})
+	}
+}
+
+func inheritanceBenchmarkInput(shape string, size int) *document.Node {
+	value := func(text string) *document.Node { return &document.Node{Kind: document.String, Text: text} }
+	mapping := func(entries ...document.Entry) *document.Node {
+		return &document.Node{Kind: document.Mapping, Mapping: entries}
+	}
+	entry := func(key string, node *document.Node) document.Entry { return document.Entry{Key: key, Value: node} }
+	base := mapping(entry("value", value("base")), entry("settings", mapping(entry("enabled", &document.Node{Kind: document.Boolean, Text: "false"}))))
+	if shape == "nested" {
+		base = mapping(entry("settings", mapping(entry("first", mapping(entry("value", value("base")))), entry("second", mapping(entry("extends", value("first")))))))
+	}
+	definitions := make([]document.Entry, 0, size+1)
+	definitions = append(definitions, entry("item-0", base))
+	for index := 1; index <= size; index++ {
+		parent := "item-0"
+		if shape == "chain" {
+			parent = "item-" + strconv.Itoa(index-1)
+		}
+		payload := []document.Entry{entry("extends", value(parent))}
+		if shape == "nested" {
+			payload = append(payload, entry("settings", mapping(entry("first", mapping(entry("value", value("custom")))))))
+		}
+		if shape == "replacement" {
+			payload = append(payload, entry("value", value("replacement")))
+		}
+		definitions = append(definitions, entry("item-"+strconv.Itoa(index), mapping(payload...)))
+	}
+	return mapping(entry("entries", mapping(definitions...)))
+}

internal/engine/inheritance_explain.go

@@ -0,0 +1,60 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+func appendInheritanceExplanation(explanation *Explanation, working **document.Node, query Path, operations []inheritanceOperation, secrets map[string]bool) {
+	for _, operation := range operations {
+		changes := projectedInheritanceChanges(query, operation)
+		if operation.control != nil {
+			if !pathsIntersect(operation.control.path, query) {
+				continue
+			}
+			before := valueSnapshot(*working, query, secrets)
+			replaceHistoryPath(working, operation.control.path, nil)
+			after := valueSnapshot(*working, query, secrets)
+			if len(changes) != 0 {
+				explanation.History = append(explanation.History, ValueChange{
+					Before: before, After: after, Origin: cloneOrigin(changes[0].Origin),
+					Dependencies: clonePaths(changes[0].Dependencies),
+				})
+			}
+			continue
+		}
+		if operation.transfer == nil {
+			continue
+		}
+		before := valueSnapshot(*working, query, secrets)
+		replaceHistoryPath(working, operation.transfer.to, operation.parent)
+		afterParent := valueSnapshot(*working, query, secrets)
+		parent, parentPresent := transferredOperationOrigin(operation, operationPathForQuery(operation.transfer.to, query))
+		if pathsIntersect(operation.transfer.to, query) && parentPresent {
+			parent.Operation = ChangeInheritance
+			parent.Inheritance = projectInheritanceSteps(parent)
+			explanation.History = append(explanation.History, ValueChange{
+				Before: before, After: afterParent, Origin: parent,
+				Dependencies: []Path{operation.transfer.from.clone(), operation.transfer.reference.clone()},
+			})
+		}
+		replaceHistoryPath(working, operation.transfer.to, operation.effective)
+		if !pathsIntersect(operation.transfer.to, query) {
+			continue
+		}
+		afterChild := valueSnapshot(*working, query, secrets)
+		if len(changes) != 0 {
+			last := changes[len(changes)-1]
+			if !parentPresent || len(changes) > 1 {
+				explanation.History = append(explanation.History, ValueChange{
+					Before: afterParent, After: afterChild, Origin: cloneOrigin(last.Origin),
+					Dependencies: clonePaths(last.Dependencies),
+				})
+			}
+		}
+	}
+}
+
+func operationPathForQuery(root, query Path) Path {
+	if pathContains(query, root) {
+		return root
+	}
+	return query
+}

internal/engine/inheritance_history.go

@@ -0,0 +1,170 @@
+package engine
+
+import (
+	"sort"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type inheritanceHistoryIndex struct {
+	paths pathPrefixIndex
+}
+
+func newInheritanceHistoryIndex(operations []inheritanceOperation) *inheritanceHistoryIndex {
+	if len(operations) == 0 {
+		return nil
+	}
+	index := &inheritanceHistoryIndex{}
+	for operationID, operation := range operations {
+		if operation.control != nil {
+			index.paths.insert(operation.control.path.String(), operationID)
+			continue
+		}
+		if operation.transfer != nil {
+			index.paths.insert(operation.transfer.to.String(), operationID)
+		}
+	}
+	return index
+}
+
+func (index *inheritanceHistoryIndex) matchingOperations(path Path, operations []inheritanceOperation) []inheritanceOperation {
+	if index == nil {
+		return operations
+	}
+	var ids []int
+	seen := make(map[int]struct{})
+	collect := func(id int) {
+		if _, found := seen[id]; found {
+			return
+		}
+		seen[id] = struct{}{}
+		ids = append(ids, id)
+	}
+	pointer := path.String()
+	index.paths.visitAncestors(pointer, collect)
+	index.paths.visitDescendants(pointer, collect)
+	sort.Ints(ids)
+	matching := make([]inheritanceOperation, 0, len(ids))
+	for _, id := range ids {
+		if id >= 0 && id < len(operations) {
+			matching = append(matching, operations[id])
+		}
+	}
+	return matching
+}
+
+// Retain source history at original paths. Structural operations are held once
+// per relationship and projected onto a requested destination path only when
+// History or Explain is queried.
+func applyInheritanceMetadata(metadata *Metadata, raw *document.Node, rawOrigins map[string]Origin, overlays []overlayRecord, expansion *inheritanceScope) {
+	if metadata == nil || expansion == nil {
+		return
+	}
+	sourceView := Metadata{
+		fields:  make(map[string]FieldInfo),
+		history: metadata.history,
+	}
+	walkDocument(raw, Path{}, func(path Path, node *document.Node) {
+		origin, found := rawOrigins[path.String()]
+		if !found {
+			origin = Origin{Layer: LayerBase, Locations: []Location{publicLocation(node.Location)}}
+		}
+		sourceView.fields[path.String()] = FieldInfo{Explicit: true, Null: node.Kind == document.Null, Origin: cloneOrigin(origin)}
+	})
+	applyOverlayMetadata(&sourceView, overlays)
+	metadata.history = sourceView.history
+	metadata.inheritance = append([]inheritanceOperation(nil), expansion.operations...)
+}
+
+func historyWithInheritance(path Path, stored []Change, operations []inheritanceOperation) []Change {
+	split := len(stored)
+	for index, change := range stored {
+		if change.Origin.Layer == LayerResolution {
+			split = index
+			break
+		}
+	}
+	history := cloneChanges(stored[:split])
+	for _, operation := range operations {
+		for _, change := range projectedInheritanceChanges(path, operation) {
+			if len(history) != 0 {
+				change.Previous = []Origin{cloneOrigin(history[len(history)-1].Origin)}
+			}
+			history = append(history, change)
+		}
+	}
+	for _, change := range stored[split:] {
+		cloned := cloneChange(change)
+		if len(history) != 0 {
+			cloned.Previous = []Origin{cloneOrigin(history[len(history)-1].Origin)}
+		}
+		history = append(history, cloned)
+	}
+	return history
+}
+
+func projectedInheritanceChanges(query Path, operation inheritanceOperation) []Change {
+	if operation.control != nil {
+		if !pathsIntersect(operation.control.path, query) {
+			return nil
+		}
+		origin := cloneOrigin(operation.controlOrigin)
+		origin.Operation = ChangeInheritance
+		origin.Inheritance = projectInheritanceSteps(origin)
+		return []Change{{Path: operation.control.path.clone(), Origin: origin,
+			Dependencies: []Path{operation.control.path.clone()}}}
+	}
+	if operation.transfer == nil || !pathsIntersect(operation.transfer.to, query) {
+		return nil
+	}
+	path := query
+	if pathContains(query, operation.transfer.to) {
+		path = operation.transfer.to
+	}
+	var changes []Change
+	parent, parentFound := transferredOperationOrigin(operation, path)
+	if parentFound {
+		parent.Operation = ChangeInheritance
+		parent.Inheritance = projectInheritanceSteps(parent)
+		changes = append(changes, Change{
+			Path: path.clone(), Origin: parent,
+			Dependencies: []Path{operation.transfer.from.clone(), operation.transfer.reference.clone()},
+		})
+	}
+	effective, effectiveFound := operation.effectiveOrigins[path.String()]
+	if effectiveFound {
+		effective = cloneOrigin(effective)
+		effective.Operation = ChangeInheritance
+	}
+	if effectiveFound && (!parentFound || !originContainedBy(effective, parent)) {
+		effective.Inheritance = projectInheritanceSteps(effective)
+		changes = append(changes, Change{
+			Path: path.clone(), Origin: effective,
+			Dependencies: []Path{operation.transfer.reference.clone()},
+		})
+	} else if parentFound && !effectiveFound {
+		removed := cloneOrigin(operation.effectiveOrigins[operation.transfer.to.String()])
+		removed.Operation = ChangeInheritance
+		removed.Inheritance = projectInheritanceSteps(removed)
+		changes = append(changes, Change{
+			Path: operation.transfer.to.clone(), Origin: removed,
+			Dependencies: []Path{operation.transfer.reference.clone()},
+		})
+	}
+	return changes
+}
+
+func transferredOperationOrigin(operation inheritanceOperation, destination Path) (Origin, bool) {
+	relative, ok := relativeHistoryPath(operation.transfer.to, destination)
+	if !ok {
+		return Origin{}, false
+	}
+	source := appendInheritancePath(operation.transfer.from, relative)
+	origin, found := operation.parentOrigins[source.String()]
+	if !found {
+		return Origin{}, false
+	}
+	transferred := transferredInheritanceOrigins(map[string]Origin{source.String(): origin},
+		operation.transfer.from, operation.transfer.to, *operation.transfer)
+	return transferred[destination.String()], true
+}

internal/engine/inheritance_history_test.go

@@ -0,0 +1,410 @@
+package engine
+
+import (
+	"bytes"
+	"reflect"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestNestedInheritanceHistoryRetainsUnresolvedTemplates(t *testing.T) {
+	input := `presets:
+  base:
+    settings:
+      primary: {width: 10}
+      secondary: {extends: primary}
+  custom:
+    extends: base
+    settings:
+      primary: {width: 20}
+  grandchild:
+    extends: custom
+    settings:
+      primary: {width: 30}
+`
+	loaded, err := Load[map[string]any](Bytes("nested", []byte(input)), testSecretBinding())
+	if err != nil {
+		t.Fatal(err)
+	}
+	for name, width := range map[string]string{"custom": "20", "grandchild": "30"} {
+		t.Run(name, func(t *testing.T) {
+			root := Keys("presets", name)
+			value, _ := loaded.Inspection.Explain(root.Key("settings").Key("secondary").Key("width"))
+			introduced := false
+			for _, change := range value.History {
+				if change.Origin.Operation == ChangeInheritance && !change.Before.Present && change.After.YAML == width+"\n" {
+					introduced = true
+				}
+			}
+			if !introduced || value.Effective.YAML != width+"\n" {
+				t.Fatalf("missing nested introduction: %#v", value)
+			}
+			controlPath := root.Key("settings").Key("secondary").Key("extends")
+			control, _ := loaded.Inspection.Explain(controlPath)
+			arrived, consumed := false, false
+			for _, change := range control.History {
+				arrived = arrived || change.After.YAML == "primary\n"
+				consumed = consumed || (change.Before.YAML == "primary\n" && !change.After.Present)
+			}
+			if !arrived || !consumed || control.Effective.Present {
+				t.Fatalf("missing control history: %#v", control)
+			}
+			container, _ := loaded.Inspection.Explain(root)
+			unresolved := false
+			for _, change := range container.History {
+				if change.Origin.Operation == ChangeInheritance && strings.Contains(change.After.YAML, "extends: primary") {
+					unresolved = true
+				}
+			}
+			if !unresolved || strings.Contains(container.Effective.YAML, "extends:") {
+				t.Fatalf("invalid enclosing snapshot: %#v", container)
+			}
+			if _, present := loaded.Inspection.Metadata().Lookup(controlPath); present {
+				t.Fatal("control is effective")
+			}
+			if len(loaded.Inspection.Metadata().History(controlPath)) != len(control.History) {
+				t.Fatal("metadata and explanation disagree")
+			}
+		})
+	}
+}
+
+func TestNestedInheritanceHistoryInsideSequence(t *testing.T) {
+	input := `base:
+  items:
+    - first: {value: inherited}
+      second: {extends: first}
+child: {extends: base}
+`
+	loaded, err := Load[map[string]any](Bytes("sequence", []byte(input)), testSecretBinding())
+	if err != nil {
+		t.Fatal(err)
+	}
+	path := Keys("child", "items").Index(0).Key("second")
+	control, _ := loaded.Inspection.Explain(path.Key("extends"))
+	consumed := false
+	for _, change := range control.History {
+		consumed = consumed || (change.Before.YAML == "first\n" && !change.After.Present)
+	}
+	if !consumed || control.Effective.Present {
+		t.Fatalf("sequence control history = %#v", control)
+	}
+	value, _ := loaded.Inspection.Explain(path.Key("value"))
+	introduced := false
+	for _, change := range value.History {
+		introduced = introduced || (!change.Before.Present && change.After.YAML == "inherited\n")
+	}
+	if !introduced || value.Effective.YAML != "inherited\n" {
+		t.Fatalf("sequence value history = %#v", value)
+	}
+}
+
+func inheritanceInspectionForTest(t *testing.T, sources ...*document.Node) (*Inspection, *inheritanceScope) {
+	t.Helper()
+	composed, diagnostics := composeDocuments(sources...)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	return inheritanceInspectionFromComposition(t, composed)
+}
+
+func inheritanceInspectionFromComposition(t *testing.T, composed *composedDocument) (*Inspection, *inheritanceScope) {
+	t.Helper()
+	expanded, diagnostics := expandInheritanceWithOrigins(composed.root, composed.origins, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	baseline := &document.Node{Kind: document.Mapping}
+	presence := make(map[string]inputPresence)
+	collectPresence(expanded.root, Path{}, presence)
+	decoded := decodedConfiguration{
+		baseline: baseline, final: expanded.root, presence: presence, origins: expanded.origins,
+	}
+	metadata := metadataFromDecoded(decoded)
+	applyInheritanceMetadata(&metadata, composed.root, composed.origins, composed.overlays, expanded)
+	finalizeMetadataPaths(&metadata, expanded.root, composed.applications, baseline, composed.root)
+	return &Inspection{
+		metadata: metadata, baseline: baseline, input: expanded.root,
+		inputOrigins: expanded.origins, final: expanded.root,
+		applications: composed.applications, inheritance: expanded.operations,
+	}, expanded
+}
+
+func TestInheritanceHistoryFollowsFinalParentAfterSourceOverlay(t *testing.T) {
+	base, diagnostics := composeDocuments(parseCompositionDocument(t, "source-1", "parent: {value: original}\nchild: {extends: parent}\n"))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	assembly := newAssemblyAccumulator(base)
+	if diagnostics := assembly.applyOverlay(parseCompositionDocument(t, "source-2", "parent: {value: latest}\n"), nil, nil, LayerOverlay); len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	inspection, _ := inheritanceInspectionFromComposition(t, assembly.current)
+	path := Keys("child", "value")
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.YAML != "latest\n" ||
+		len(explanation.History) == 0 || explanation.History[len(explanation.History)-1].Origin.Locations[0].SourceID != "source-2" {
+		t.Fatalf("overlaid parent contribution = %#v, %v", explanation, found)
+	}
+	parentHistory := inspection.Metadata().History(Keys("parent", "value"))
+	if len(parentHistory) < 2 || parentHistory[0].Origin.Layer != LayerBase ||
+		parentHistory[len(parentHistory)-1].Origin.Layer != LayerOverlay {
+		t.Fatalf("parent source history = %#v", parentHistory)
+	}
+}
+
+func TestInheritanceHistoryPreservesCancellationAndSupersededDirective(t *testing.T) {
+	base, diagnostics := composeDocuments(parseCompositionDocument(t, "source-1", "parent: {value: one}\nchild: {extends: missing, value: own}\n"))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	assembly := newAssemblyAccumulator(base)
+	if diagnostics := assembly.applyOverlay(parseCompositionDocument(t, "source-2", "child: {extends: null}\n"), nil, nil, LayerOverlay); len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	inspection, _ := inheritanceInspectionFromComposition(t, assembly.current)
+	control := Keys("child", inheritanceKey)
+	history := inspection.Metadata().History(control)
+	if len(history) < 3 || history[0].Origin.Locations[0].SourceID != "source-1" ||
+		history[1].Origin.Locations[0].SourceID != "source-2" ||
+		history[len(history)-1].Origin.Operation != ChangeInheritance {
+		t.Fatalf("superseded and cancelled directive history = %#v", history)
+	}
+	explanation, found := inspection.Explain(control)
+	if !found || explanation.Effective.Present || len(explanation.History) < 3 ||
+		explanation.History[len(explanation.History)-1].After.Present {
+		t.Fatalf("cancelled directive explanation = %#v, %v", explanation, found)
+	}
+	value, found := inspection.Explain(Keys("child", "value"))
+	if !found || value.Effective.YAML != "own\n" {
+		t.Fatalf("cancelled relationship changed own value = %#v, %v", value, found)
+	}
+}
+
+func TestInheritanceHistoryProjectsNestedDestinationValues(t *testing.T) {
+	inspection, _ := inheritanceInspectionForTest(t, parseCompositionDocument(t, "source-1", `base:
+  settings:
+    primary: {value: old}
+    secondary: {extends: primary}
+custom:
+  extends: base
+  settings:
+    primary: {value: new}
+`))
+	path := Keys("custom", "settings", "secondary", "value")
+	history := inspection.Metadata().History(path)
+	structural := 0
+	for _, change := range history {
+		if change.Origin.Operation == ChangeInheritance {
+			structural++
+		}
+	}
+	if structural == 0 {
+		t.Fatalf("nested destination history = %#v", history)
+	}
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.YAML != "new\n" ||
+		len(explanation.Origin.Inheritance) == 0 ||
+		explanation.Origin.Inheritance[len(explanation.Origin.Inheritance)-1].To.String() != path.String() {
+		t.Fatalf("nested destination explanation = %#v, %v", explanation, found)
+	}
+	for _, change := range explanation.History {
+		if change.Origin.Operation == ChangeDecoding {
+			t.Fatalf("nested inheritance was attributed to decoding: %#v", explanation.History)
+		}
+	}
+}
+
+func TestInheritanceHistoryShowsParentAndEqualChildContribution(t *testing.T) {
+	inspection, _ := inheritanceInspectionForTest(t,
+		parseCompositionDocument(t, "source-1", "parent: {value: same}\n"),
+		parseCompositionDocument(t, "source-2", "child: {extends: parent, value: same}\n"))
+	path := Keys("child", "value")
+	history := inspection.Metadata().History(path)
+	var structural []Change
+	for _, change := range history {
+		if change.Origin.Operation == ChangeInheritance {
+			structural = append(structural, change)
+		}
+	}
+	if len(structural) != 2 || structural[0].Origin.Locations[0].SourceID != "source-1" ||
+		structural[1].Origin.Locations[0].SourceID != "source-2" ||
+		len(structural[0].Dependencies) != 2 || structural[0].Dependencies[0].String() != "/parent" ||
+		len(structural[0].Origin.Inheritance) != 1 {
+		t.Fatalf("equal-valued structural history = %#v", history)
+	}
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.YAML != "same\n" {
+		t.Fatalf("explanation = %#v, %v", explanation, found)
+	}
+	var equalEvents int
+	for _, change := range explanation.History {
+		if change.Origin.Operation == ChangeDecoding {
+			t.Fatalf("inheritance was attributed to decoding: %#v", explanation.History)
+		}
+		if change.Origin.Operation == ChangeInheritance && change.Before.YAML == change.After.YAML {
+			equalEvents++
+		}
+	}
+	if equalEvents != 2 {
+		t.Fatalf("equal parent and child operations = %#v", explanation.History)
+	}
+	parentHistory := inspection.Metadata().History(Keys("parent", "value"))
+	for _, change := range parentHistory {
+		if change.Origin.Operation == ChangeInheritance {
+			t.Fatalf("source definition acquired a destination operation: %#v", parentHistory)
+		}
+	}
+}
+
+func TestInheritanceHistoryRetainsControlsAndDisplacedPaths(t *testing.T) {
+	inspection, _ := inheritanceInspectionForTest(t,
+		parseCompositionDocument(t, "source-1", "parent: {branch: {detail: inherited}}\n"),
+		parseCompositionDocument(t, "source-2", "child: {extends: parent, branch: replaced}\n"))
+	control := Keys("child", inheritanceKey)
+	controlHistory := inspection.Metadata().History(control)
+	if len(controlHistory) < 2 || controlHistory[len(controlHistory)-1].Origin.Operation != ChangeInheritance {
+		t.Fatalf("control history = %#v", controlHistory)
+	}
+	controlExplanation, found := inspection.Explain(control)
+	if !found || controlExplanation.Effective.Present ||
+		controlExplanation.History[len(controlExplanation.History)-1].After.Present {
+		t.Fatalf("consumed control = %#v, %v", controlExplanation, found)
+	}
+	path := Keys("child", "branch", "detail")
+	if _, found := inspection.Metadata().Lookup(path); found {
+		t.Fatal("displaced descendant remained effective")
+	}
+	listed := false
+	for _, historical := range inspection.Metadata().HistoricalPaths() {
+		if historical.String() == path.String() {
+			listed = true
+		}
+	}
+	if !listed {
+		t.Fatal("displaced inherited descendant was not enumerable")
+	}
+	history := inspection.Metadata().History(path)
+	if len(history) < 2 || history[len(history)-1].Origin.Operation != ChangeInheritance {
+		t.Fatalf("displaced descendant history = %#v", history)
+	}
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.Present {
+		t.Fatalf("displaced descendant explanation = %#v, %v", explanation, found)
+	}
+	var sawInherited, sawRemoval bool
+	for _, change := range explanation.History {
+		if change.Origin.Operation != ChangeInheritance {
+			continue
+		}
+		sawInherited = sawInherited || change.After.Present
+		sawRemoval = sawRemoval || (change.Before.Present && !change.After.Present)
+	}
+	if !sawInherited || !sawRemoval {
+		t.Fatalf("displaced value sequence = %#v", explanation.History)
+	}
+	var rendered bytes.Buffer
+	if err := inspection.WriteExplanation(&rendered, path); err != nil ||
+		!strings.Contains(rendered.String(), "structural inheritance") {
+		t.Fatalf("rendered structural explanation = %q, %v", rendered.String(), err)
+	}
+}
+
+func TestIndexedInheritanceHistoryMatchesOperationScan(t *testing.T) {
+	inspection, _ := inheritanceInspectionForTest(t,
+		parseCompositionDocument(t, "source-1", "parent: {branch: {detail: inherited, retained: present}}\n"),
+		parseCompositionDocument(t, "source-2", "child: {extends: parent, branch: replaced}\n"),
+		parseCompositionDocument(t, "source-3", "sibling: {extends: parent}\n"))
+	metadata := inspection.Metadata()
+	if len(metadata.inheritance) < 4 {
+		t.Fatalf("inheritance operations = %d, want multiple control and transfer scopes", len(metadata.inheritance))
+	}
+	queries := []Path{
+		Keys("child"),
+		Keys("child", inheritanceKey),
+		Keys("child", "branch", "detail"),
+		Keys("child", "branch"),
+		Keys("sibling", "branch", "detail"),
+		Path{},
+	}
+	for _, path := range queries {
+		if !metadata.hasHistoricalPath(path) {
+			continue
+		}
+		stored := metadata.history[path.String()]
+		want := historyWithInheritance(path, stored, metadata.inheritance)
+		if got := metadata.History(path); !reflect.DeepEqual(got, want) {
+			t.Errorf("indexed history for %s differs from operation scan:\n got: %#v\nwant: %#v", path, got, want)
+		}
+		unprepared := Metadata{history: metadata.history, historicalPaths: metadata.historicalPaths, inheritance: metadata.inheritance}
+		if got := unprepared.History(path); !reflect.DeepEqual(got, want) {
+			t.Errorf("unprepared history for %s differs from operation scan:\n got: %#v\nwant: %#v", path, got, want)
+		}
+	}
+
+	removed := Keys("child", "branch", "detail")
+	first := metadata.History(removed)
+	if len(first) == 0 || len(first[0].Path.segments) == 0 {
+		t.Fatalf("removed-path history = %#v", first)
+	}
+	first[0].Path.segments[0].key = "mutated"
+	second := cloneMetadata(metadata).History(removed)
+	want := historyWithInheritance(removed, metadata.history[removed.String()], metadata.inheritance)
+	if !reflect.DeepEqual(second, want) {
+		t.Fatalf("returned history mutation affected a repeated or cloned query:\n got: %#v\nwant: %#v", second, want)
+	}
+}
+
+func TestInheritanceHistoryPrecedesApplicationResolution(t *testing.T) {
+	inspection, _ := inheritanceInspectionForTest(t,
+		parseCompositionDocument(t, "source-1", "parent: {value: original}\nchild: {extends: parent}\n"))
+	path := Keys("child", "value")
+	replacement := &document.Node{Kind: document.String, Text: "derived"}
+	snapshot := resolutionSnapshot{
+		change: Change{Path: path, Origin: Origin{Layer: LayerResolution, Operation: ChangeNormalization}},
+		after:  replacement,
+	}
+	final := document.Clone(inspection.final)
+	replaceHistoryPath(&final, path, replacement)
+	inspection.final = final
+	inspection.resolution = []resolutionSnapshot{snapshot}
+	inspection.resolutionReplay = coalescedResolutionSnapshots(inspection.resolution)
+	inspection.metadata.history[path.String()] = append(inspection.metadata.history[path.String()], snapshot.change)
+	inspection.metadata.fields[path.String()] = FieldInfo{Explicit: true, Origin: snapshot.change.Origin}
+	history := inspection.Metadata().History(path)
+	if len(history) < 2 || history[len(history)-2].Origin.Operation != ChangeInheritance ||
+		history[len(history)-1].Origin.Operation != ChangeNormalization {
+		t.Fatalf("resolution order = %#v", history)
+	}
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.YAML != "derived\n" ||
+		explanation.History[len(explanation.History)-1].Origin.Operation != ChangeNormalization {
+		t.Fatalf("resolved explanation = %#v, %v", explanation, found)
+	}
+}
+
+func TestHistoryNodeBudgetCountsSharedInheritanceEvidenceOnce(t *testing.T) {
+	shared := &document.Node{Kind: document.String, Text: "shared"}
+	extra := &document.Node{Kind: document.String, Text: "extra"}
+	expansion := &inheritanceScope{
+		root:         shared,
+		operations:   []inheritanceOperation{{parent: shared, effective: shared}},
+		replacements: []inheritanceReplacement{{before: shared, after: extra}},
+	}
+	if !historyNodesWithin(2, nil, nil, nil, nil, nil, expansion) ||
+		historyNodesWithin(1, nil, nil, nil, nil, nil, expansion) {
+		t.Fatal("shared evidence node accounting is incorrect")
+	}
+}
+
+func TestInheritanceExpansionEnforcesRetainedHistoryBudget(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", "parent: {value: original}\nchild: {extends: parent, value: replacement}\n")
+	if _, diagnostics := expandInheritanceWithBudgets(input, nil, defaultProjectionLimits(), 2); len(diagnostics) != 1 || diagnostics[0].Code != CodeExplainLimitExceeded {
+		t.Fatalf("retained history limit = %#v", diagnostics)
+	}
+	if _, diagnostics := expandInheritanceWithBudgets(input, nil, defaultProjectionLimits(), 100); len(diagnostics) != 0 {
+		t.Fatalf("sufficient retained history budget = %#v", diagnostics)
+	}
+}

internal/engine/inheritance_load.go

@@ -0,0 +1,67 @@
+package engine
+
+import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+
+func containsInheritanceDirective(root *document.Node) bool {
+	var visit func(*document.Node) bool
+	visit = func(node *document.Node) bool {
+		if node == nil {
+			return false
+		}
+		switch node.Kind {
+		case document.Mapping:
+			for _, entry := range node.Mapping {
+				if entry.Key == inheritanceKey || visit(entry.Value) {
+					return true
+				}
+			}
+		case document.Sequence:
+			for _, child := range node.Sequence {
+				if visit(child) {
+					return true
+				}
+			}
+		}
+		return false
+	}
+	return visit(root)
+}
+
+func inheritanceVersionDiagnostic(rule *VersionRule, expanded *inheritanceScope) *Diagnostic {
+	if rule == nil || expanded == nil {
+		return nil
+	}
+	for _, operation := range expanded.operations {
+		var location Location
+		var affected bool
+		if operation.control != nil {
+			affected = pathsIntersect(operation.control.path, rule.Path)
+			if len(operation.control.locations) != 0 {
+				location = operation.control.locations[0]
+			}
+		} else if operation.transfer != nil {
+			if relative, within := relativeHistoryPath(operation.transfer.to, rule.Path); within {
+				_, affected = documentAtPath(operation.parent, relative)
+			} else if pathContains(rule.Path, operation.transfer.to) {
+				affected = true
+			}
+			if len(operation.transfer.locations) != 0 {
+				location = operation.transfer.locations[0]
+			}
+		}
+		if affected {
+			return &Diagnostic{
+				Code: CodeInheritanceVersion, Severity: SeverityError, Path: rule.Path.clone(),
+				Message: "inheritance cannot change the schema-version path", Primary: &location,
+			}
+		}
+	}
+	return nil
+}
+
+func inheritanceOperations(expanded *inheritanceScope) []inheritanceOperation {
+	if expanded == nil {
+		return nil
+	}
+	return expanded.operations
+}

internal/engine/inheritance_origin.go

@@ -0,0 +1,158 @@
+package engine
+
+import (
+	"slices"
+	"strconv"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func directInheritanceOrigins(node *document.Node, root Path, input map[string]Origin) map[string]Origin {
+	result := make(map[string]Origin)
+	walkDocument(node, root, func(path Path, value *document.Node) {
+		origin, found := input[path.String()]
+		if !found {
+			origin = Origin{Layer: LayerBase, Locations: []Location{publicLocation(value.Location)}}
+		}
+		origin = cloneOrigin(origin)
+		if len(origin.contributors) == 0 {
+			origin.contributors = make([]inheritanceContributor, len(origin.Locations))
+			for index, location := range origin.Locations {
+				origin.contributors[index] = inheritanceContributor{
+					source: path.clone(), location: location,
+				}
+			}
+		}
+		result[path.String()] = origin
+	})
+	return result
+}
+
+func transferredInheritanceOrigins(parent map[string]Origin, from, to Path, control inheritanceTransfer) map[string]Origin {
+	result := make(map[string]Origin, len(parent))
+	transfer := control // one immutable edge shared by every matching contributor
+	links := make(map[*inheritanceTrace]*inheritanceTrace)
+	for pointer, origin := range parent {
+		path, err := ParsePath(pointer)
+		if err != nil {
+			continue
+		}
+		relative, ok := relativeHistoryPath(from, path)
+		if !ok {
+			continue
+		}
+		destination := appendInheritancePath(to, relative)
+		origin = cloneOrigin(origin)
+		for index, contributor := range origin.contributors {
+			link, found := links[contributor.trace]
+			if !found {
+				link = &inheritanceTrace{previous: contributor.trace, transfer: &transfer}
+				links[contributor.trace] = link
+			}
+			origin.contributors[index].trace = link
+		}
+		result[destination.String()] = origin
+	}
+	return result
+}
+
+func appendInheritancePath(root, relative Path) Path {
+	return Path{segments: append(clonePathSegments(root.segments, len(relative.segments)), relative.segments...)}
+}
+
+func mergeInheritanceOrigin(origins map[string]Origin, path Path, incoming Origin) {
+	pointer := path.String()
+	current := origins[pointer]
+	current.Layer = incoming.Layer
+	current.Locations = append(current.Locations, incoming.Locations...)
+	current.contributors = append(current.contributors, incoming.contributors...)
+	origins[pointer] = current
+}
+
+func copyInheritanceSubtree(target, incoming map[string]Origin, node *document.Node, root Path) {
+	walkDocument(node, root, func(path Path, _ *document.Node) {
+		target[path.String()] = cloneOrigin(incoming[path.String()])
+	})
+}
+
+type inheritanceStepKey struct {
+	from      string
+	to        string
+	reference string
+	locations string
+}
+
+func inheritanceStepKeyForPaths(from, to, reference Path, locations []Location) inheritanceStepKey {
+	return inheritanceStepKey{
+		from: from.String(), to: to.String(), reference: reference.String(),
+		locations: inheritanceLocationsKey(locations),
+	}
+}
+
+func inheritanceLocationsKey(locations []Location) string {
+	encoded := strconv.AppendInt(nil, int64(len(locations)), 10)
+	encoded = append(encoded, ';')
+	for _, location := range locations {
+		encoded = strconv.AppendInt(encoded, int64(len(location.SourceID)), 10)
+		encoded = append(encoded, ':')
+		encoded = append(encoded, location.SourceID...)
+		encoded = append(encoded, ';')
+		encoded = strconv.AppendInt(encoded, int64(location.Line), 10)
+		encoded = append(encoded, ';')
+		encoded = strconv.AppendInt(encoded, int64(location.Column), 10)
+		encoded = append(encoded, ';')
+	}
+	return string(encoded)
+}
+
+// Materialize concrete steps only for an inspection record. The linked private
+// traces remain compact when one parent supplies many descendant values.
+func projectInheritanceSteps(origin Origin) []InheritanceStep {
+	steps := cloneInheritanceSteps(origin.Inheritance)
+	seen := make(map[inheritanceStepKey]struct{}, len(steps))
+	for _, step := range steps {
+		seen[inheritanceStepKeyForPaths(step.From, step.To, step.Reference, step.Locations)] = struct{}{}
+	}
+	for _, contributor := range origin.contributors {
+		var reverse []*inheritanceTrace
+		for link := contributor.trace; link != nil; link = link.previous {
+			reverse = append(reverse, link)
+		}
+		current := contributor.source
+		for index := len(reverse) - 1; index >= 0; index-- {
+			transfer := reverse[index].transfer
+			relative, ok := relativeHistoryPath(transfer.from, current)
+			if !ok {
+				continue
+			}
+			next := appendInheritancePath(transfer.to, relative)
+			key := inheritanceStepKeyForPaths(current, next, transfer.reference, transfer.locations)
+			if _, found := seen[key]; !found {
+				seen[key] = struct{}{}
+				steps = append(steps, InheritanceStep{
+					From: current.clone(), To: next.clone(), Reference: transfer.reference.clone(),
+					Locations: cloneLocations(transfer.locations),
+				})
+			}
+			current = next
+		}
+	}
+	return steps
+}
+
+func sameInheritanceTrace(left, right *inheritanceTrace) bool {
+	for left != nil && right != nil {
+		if left == right {
+			return true
+		}
+		l, r := left.transfer, right.transfer
+		if !slices.Equal(l.from.segments, r.from.segments) ||
+			!slices.Equal(l.to.segments, r.to.segments) ||
+			!slices.Equal(l.reference.segments, r.reference.segments) ||
+			!slices.Equal(l.locations, r.locations) {
+			return false
+		}
+		left, right = left.previous, right.previous
+	}
+	return left == nil && right == nil
+}

internal/engine/inheritance_origin_test.go

@@ -0,0 +1,302 @@
+package engine
+
+import (
+	"io/fs"
+	"reflect"
+	"slices"
+	"testing"
+	"testing/fstest"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestInheritedOriginsKeepSourcePathsPresenceAndIndependentReplacement(t *testing.T) {
+	parent := parseCompositionDocument(t, "source-1", `parent:
+  enabled: false
+  count: 0
+  optional: null
+  items: [{file: message.txt}]
+  settings: {shared: parent}
+`)
+	child := parseCompositionDocument(t, "source-2", `child:
+  extends: parent
+  count: 3
+  settings: {own: child}
+`)
+	composed, diagnostics := composeDocuments(parent, child)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	result, diagnostics := expandInheritanceWithOrigins(composed.root, composed.origins, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	for _, key := range []string{"enabled", "optional"} {
+		path := Keys("child", key)
+		value, found := documentAtPath(result.root, path)
+		origin, hasOrigin := result.origins[path.String()]
+		if !found || !hasOrigin || value == nil || len(origin.Locations) != 1 || origin.Locations[0].SourceID != "source-1" ||
+			len(projectInheritanceSteps(origin)) != 1 {
+			t.Fatalf("inherited %s = %#v, %#v", key, value, origin)
+		}
+	}
+	count := result.origins["/child/count"]
+	if len(count.Locations) != 1 || count.Locations[0].SourceID != "source-2" || len(projectInheritanceSteps(count)) != 0 {
+		t.Fatalf("direct replacement origin = %#v", count)
+	}
+	if len(result.replacements) != 1 || result.replacements[0].path.String() != "/child/count" ||
+		result.replacements[0].before.Text != "0" || result.replacements[0].after.Text != "3" ||
+		len(result.replacements[0].previous) != 1 ||
+		len(projectInheritanceSteps(result.replacements[0].previous[0].origin)) != 1 {
+		t.Fatalf("displaced inheritance = %#v", result.replacements)
+	}
+	settings := result.origins["/child/settings"]
+	if len(settings.Locations) != 2 || settings.Locations[0].SourceID != "source-1" ||
+		settings.Locations[1].SourceID != "source-2" || len(projectInheritanceSteps(settings)) != 1 {
+		t.Fatalf("composite mapping origin = %#v", settings)
+	}
+	if originContainedBy(result.origins["/child/enabled"], result.origins["/parent/enabled"]) ||
+		originContainedBy(result.origins["/parent/enabled"], result.origins["/child/enabled"]) {
+		t.Fatal("direct and inherited evidence collapsed despite matching source coordinates")
+	}
+	original := result.origins["/child/enabled"]
+	detached := cloneOrigin(original)
+	detached.Locations[0].SourceID = "changed"
+	detached.contributors[0].source.segments[0].key = "changed"
+	stepsCopy := projectInheritanceSteps(original)
+	stepsCopy[0].Locations[0].SourceID = "changed"
+	if original.Locations[0].SourceID != "source-1" ||
+		original.contributors[0].source.String() != "/parent/enabled" ||
+		projectInheritanceSteps(original)[0].Locations[0].SourceID != "source-2" {
+		t.Fatal("cloned origin or projected steps changed retained evidence")
+	}
+	resource := result.origins["/child/items/0/file"]
+	steps := projectInheritanceSteps(resource)
+	if len(steps) != 1 || steps[0].From.String() != "/parent/items/0/file" ||
+		steps[0].To.String() != "/child/items/0/file" || steps[0].Reference.String() != "/child/extends" ||
+		len(resource.Locations) != 1 || resource.Locations[0].SourceID != "source-1" {
+		t.Fatalf("sequence resource origin = %#v, steps = %#v", resource, steps)
+	}
+	contexts := map[string]fs.FS{
+		"source-1": fstest.MapFS{"message.txt": &fstest.MapFile{Data: []byte("from parent")}},
+		"source-2": fstest.MapFS{"message.txt": &fstest.MapFile{Data: []byte("from child")}},
+	}
+	data, err := fs.ReadFile(contexts[resource.Locations[0].SourceID], "message.txt")
+	if err != nil || string(data) != "from parent" {
+		t.Fatalf("inherited resource = %q, %v", data, err)
+	}
+}
+
+func TestOriginMembershipPreservesLocationAndSourceCoverageRules(t *testing.T) {
+	location := Location{SourceID: "source", Line: 4, Column: 2}
+	otherCoordinates := location
+	otherCoordinates.Line++
+
+	container := Origin{
+		Layer:     LayerOverlay,
+		Operation: ChangeNormalization,
+		Locations: []Location{location},
+	}
+	if !originContainedBy(container, container) {
+		t.Fatal("identical location evidence was not contained")
+	}
+	coordinateDifference := container
+	coordinateDifference.Locations = []Location{otherCoordinates}
+	if originContainedBy(coordinateDifference, container) {
+		t.Fatal("location containment ignored differing coordinates")
+	}
+
+	parent := Keys("parent")
+	child := parent.Key("child")
+	change := Change{Path: parent, Origin: container}
+	descendantOrigin := Origin{
+		Layer:     container.Layer,
+		Operation: container.Operation,
+		Locations: []Location{otherCoordinates},
+	}
+	if !originRepresentedByChange(child, descendantOrigin, change) {
+		t.Fatal("descendant source coverage rejected matching SourceID with different coordinates")
+	}
+	descendantOrigin.Locations[0].SourceID = "another-source"
+	if originRepresentedByChange(child, descendantOrigin, change) {
+		t.Fatal("descendant source coverage accepted an unrecorded SourceID")
+	}
+
+	withContributor := container
+	withContributor.contributors = []inheritanceContributor{{source: parent, location: location}}
+	if originContainedBy(withContributor, container) {
+		t.Fatal("exact location containment ignored the contributor-empty mismatch")
+	}
+	if !originRepresentedByChange(child, withContributor, change) {
+		t.Fatal("descendant source coverage was conflated with contributor containment")
+	}
+}
+
+func TestProjectedInheritanceStepsMatchScanAndRetainOwnership(t *testing.T) {
+	sourcePath := Keys("source", "a/b", "01")
+	firstLocations := []Location{{SourceID: "first;:source", Line: -1, Column: 2}}
+	secondLocations := []Location{
+		{SourceID: "second:;source", Line: 3, Column: -4},
+		{SourceID: "", Line: -5, Column: 6},
+	}
+	firstTransfer := inheritanceTransfer{
+		from: Keys("source"), to: Keys("middle"), reference: Keys("middle", "extends"), locations: firstLocations,
+	}
+	secondTransfer := inheritanceTransfer{
+		from: Keys("middle"), to: Keys("target"), reference: Keys("target", "extends"), locations: secondLocations,
+	}
+	firstTrace := &inheritanceTrace{transfer: &firstTransfer}
+	chain := &inheritanceTrace{previous: firstTrace, transfer: &secondTransfer}
+	firstStep := InheritanceStep{
+		From: sourcePath, To: Keys("middle", "a/b", "01"), Reference: firstTransfer.reference, Locations: firstLocations,
+	}
+
+	duplicateFirstTransfer := firstTransfer
+	duplicateSecondTransfer := secondTransfer
+	duplicateChain := &inheritanceTrace{
+		previous: &inheritanceTrace{transfer: &duplicateFirstTransfer},
+		transfer: &duplicateSecondTransfer,
+	}
+	reversedLocations := []Location{secondLocations[1], secondLocations[0]}
+	reorderedSecondTransfer := secondTransfer
+	reorderedSecondTransfer.locations = reversedLocations
+	reorderedChain := &inheritanceTrace{
+		previous: &inheritanceTrace{transfer: &duplicateFirstTransfer},
+		transfer: &reorderedSecondTransfer,
+	}
+	origin := Origin{
+		Inheritance: []InheritanceStep{firstStep, firstStep},
+		contributors: []inheritanceContributor{
+			{source: sourcePath, trace: chain},
+			{source: sourcePath, trace: duplicateChain},
+			{source: sourcePath, trace: reorderedChain},
+			{source: sourcePath, trace: chain},
+		},
+	}
+
+	want := projectInheritanceStepsByScan(origin)
+	got := projectInheritanceSteps(origin)
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("projected steps differ from ordered scan:\n got: %#v\nwant: %#v", got, want)
+	}
+	if len(got) != 4 || got[0].From.String() != "/source/a~1b/01" || got[0].To.String() != "/middle/a~1b/01" ||
+		!reflect.DeepEqual(got[0], got[1]) || got[2].To.String() != "/target/a~1b/01" ||
+		!reflect.DeepEqual(got[2].Locations, secondLocations) || !reflect.DeepEqual(got[3].Locations, reversedLocations) {
+		t.Fatalf("unexpected duplicate/order/escape projection: %#v", got)
+	}
+
+	got[0].From.segments[0].key = "mutated"
+	got[2].Locations[0].SourceID = "mutated"
+	if repeated := projectInheritanceSteps(origin); !reflect.DeepEqual(repeated, want) {
+		t.Fatalf("returned steps mutated retained evidence:\n got: %#v\nwant: %#v", repeated, want)
+	}
+}
+
+func TestInheritanceLocationsKeyEncodesOrderedLengthPrefixedValues(t *testing.T) {
+	if inheritanceLocationsKey(nil) != inheritanceLocationsKey([]Location{}) {
+		t.Fatal("nil and empty location lists should encode equally")
+	}
+	values := [][]Location{
+		{{SourceID: "a;1;-2;", Line: -3, Column: 4}},
+		{{SourceID: "a", Line: 1, Column: -2}, {SourceID: ";1;-2;", Line: -3, Column: 4}},
+		{{SourceID: ";1;-2;", Line: -3, Column: 4}, {SourceID: "a", Line: 1, Column: -2}},
+		{{SourceID: "é", Line: -3, Column: 4}},
+		{{SourceID: "e", Line: -3, Column: 4}},
+	}
+	keys := make(map[string]int, len(values))
+	for index, locations := range values {
+		key := inheritanceLocationsKey(locations)
+		if previous, found := keys[key]; found {
+			t.Fatalf("location lists %d and %d encoded identically", previous, index)
+		}
+		keys[key] = index
+	}
+}
+
+func projectInheritanceStepsByScan(origin Origin) []InheritanceStep {
+	steps := cloneInheritanceSteps(origin.Inheritance)
+	for _, contributor := range origin.contributors {
+		var reverse []*inheritanceTrace
+		for link := contributor.trace; link != nil; link = link.previous {
+			reverse = append(reverse, link)
+		}
+		current := contributor.source
+		for index := len(reverse) - 1; index >= 0; index-- {
+			transfer := reverse[index].transfer
+			relative, ok := relativeHistoryPath(transfer.from, current)
+			if !ok {
+				continue
+			}
+			next := appendInheritancePath(transfer.to, relative)
+			step := InheritanceStep{
+				From: current.clone(), To: next.clone(), Reference: transfer.reference.clone(),
+				Locations: cloneLocations(transfer.locations),
+			}
+			if !slices.ContainsFunc(steps, func(existing InheritanceStep) bool {
+				return existing.From.String() == step.From.String() &&
+					existing.To.String() == step.To.String() &&
+					existing.Reference.String() == step.Reference.String() &&
+					slices.Equal(existing.Locations, step.Locations)
+			}) {
+				steps = append(steps, step)
+			}
+			current = next
+		}
+	}
+	return steps
+}
+
+func TestInheritedOriginsRetainChainsAndNestedDestinations(t *testing.T) {
+	first := parseCompositionDocument(t, "source-1", `first:
+  enabled: false
+  settings:
+    primary: {value: original}
+    secondary: {extends: primary}
+`)
+	second := parseCompositionDocument(t, "source-2", `first: {other: present}
+middle: {extends: first}
+last:
+  extends: middle
+  settings:
+    primary: {value: replaced}
+`)
+	composed, diagnostics := composeDocuments(first, second)
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	result, diagnostics := expandInheritanceWithOrigins(composed.root, composed.origins, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	chain := projectInheritanceSteps(result.origins["/last/enabled"])
+	if len(chain) != 2 || chain[0].From.String() != "/first/enabled" ||
+		chain[0].To.String() != "/middle/enabled" || chain[1].From.String() != "/middle/enabled" ||
+		chain[1].To.String() != "/last/enabled" {
+		t.Fatalf("chain = %#v", chain)
+	}
+	composite := result.origins["/last"]
+	if len(composite.Locations) < 2 || len(composite.contributors) != len(composite.Locations) {
+		t.Fatalf("composite contributors = %#v", composite)
+	}
+	seenSources := make(map[string]bool)
+	for _, contributor := range composite.contributors {
+		seenSources[contributor.location.SourceID] = true
+	}
+	if !seenSources["source-1"] || !seenSources["source-2"] {
+		t.Fatalf("same-layer source contributors = %#v", composite.contributors)
+	}
+	nested := projectInheritanceSteps(result.origins["/last/settings/secondary/value"])
+	if len(nested) != 1 || nested[0].From.String() != "/last/settings/primary/value" ||
+		nested[0].To.String() != "/last/settings/secondary/value" {
+		t.Fatalf("destination-specific nested transfer = %#v", nested)
+	}
+	if got := result.origins["/last/settings/secondary/value"].Locations[0].SourceID; got != "source-2" {
+		t.Fatalf("nested replacement source = %q", got)
+	}
+	if original, found := documentAtPath(first, Keys("first", "settings", "primary", "value")); !found || original.Text != "original" {
+		t.Fatal("expansion changed its input")
+	}
+	if inherited, found := documentAtPath(result.root, Keys("last", "settings", "secondary", "value")); !found || inherited.Kind != document.String || inherited.Text != "replaced" {
+		t.Fatalf("nested inherited value = %#v, %v", inherited, found)
+	}
+}

internal/engine/inheritance_secret_test.go

@@ -0,0 +1,136 @@
+package engine
+
+import (
+	"bytes"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestInheritanceSensitivityFollowsCopiesWithoutMaskingNeighbors(t *testing.T) {
+	const canary = "INHERITED-CANARY-CREDENTIAL"
+	input := parseCompositionDocument(t, "source-1", "parent: {token: "+canary+", plain: visible}\nchild: {extends: parent}\nnext: {extends: child}\nother: {extends: parent}\n")
+	for _, classified := range []Path{Keys("parent", "token"), Keys("next", "token")} {
+		t.Run(classified.String(), func(t *testing.T) {
+			inspection, _ := inheritanceInspectionForTest(t, input)
+			inspection.secrets = classifySecretPaths(nil, func(path Path) bool {
+				return path.String() == classified.String()
+			}, inspection.metadata, inspection.final, inspection.applications, inspection.input)
+			for _, path := range []Path{Keys("parent", "token"), Keys("child", "token"), Keys("next", "token"), Keys("other", "token")} {
+				explanation, found := inspection.Explain(path)
+				if !found || !explanation.Effective.Redacted || strings.Contains(explanation.Effective.YAML, canary) {
+					t.Fatalf("sensitive copy %s = %#v, %v", path, explanation, found)
+				}
+				for _, change := range explanation.History {
+					if strings.Contains(change.Before.YAML, canary) || strings.Contains(change.After.YAML, canary) {
+						t.Fatalf("historical canary at %s: %#v", path, change)
+					}
+				}
+			}
+			plain, found := inspection.Explain(Keys("other", "plain"))
+			if !found || plain.Effective.Redacted || plain.Effective.YAML != "visible\n" {
+				t.Fatalf("unrelated field = %#v, %v", plain, found)
+			}
+			var output bytes.Buffer
+			if err := inspection.WriteEffective(&output); err != nil || strings.Contains(output.String(), canary) || !strings.Contains(output.String(), "visible") {
+				t.Fatalf("effective rendering = %q, %v", output.String(), err)
+			}
+		})
+	}
+}
+
+func TestInheritanceSensitivityPreservesDisplacedHistoryAndControl(t *testing.T) {
+	const canary = "DISPLACED-CANARY-CREDENTIAL"
+	inspection, _ := inheritanceInspectionForTest(t, parseCompositionDocument(t, "source-1", "parent: {branch: {token: "+canary+"}}\nchild: {extends: parent, branch: public}\n"))
+	inspection.secrets = classifySecretPaths(nil, func(path Path) bool {
+		return path.String() == Keys("parent", "branch", "token").String() || path.String() == Keys("child", inheritanceKey).String()
+	}, inspection.metadata, inspection.final, inspection.applications, inspection.input)
+	path := Keys("child", "branch", "token")
+	explanation, found := inspection.Explain(path)
+	if !found || explanation.Effective.Present {
+		t.Fatalf("displaced sensitive path = %#v, %v", explanation, found)
+	}
+	for _, change := range explanation.History {
+		if strings.Contains(change.Before.YAML, canary) || strings.Contains(change.After.YAML, canary) ||
+			(change.After.Present && !change.After.Redacted) {
+			t.Fatalf("displaced secret leaked: %#v", explanation.History)
+		}
+	}
+	var rendered bytes.Buffer
+	if err := inspection.WriteExplanation(&rendered, path); err != nil || strings.Contains(rendered.String(), canary) {
+		t.Fatalf("displaced explanation rendering = %q, %v", rendered.String(), err)
+	}
+	control, found := inspection.Explain(Keys("child", inheritanceKey))
+	if !found || control.Effective.Present || len(control.History) == 0 || !control.History[0].After.Redacted {
+		t.Fatalf("sensitive control history = %#v, %v", control, found)
+	}
+	info, found := inspection.Metadata().Lookup(Keys("child", "branch"))
+	if !found || len(info.Origin.Inheritance) != 0 {
+		t.Fatalf("independent replacement provenance = %#v, %v", info, found)
+	}
+}
+
+func TestInheritanceSensitivityIncludesSchemaTaggedSource(t *testing.T) {
+	type entry struct {
+		Token string `yaml:"token"`
+	}
+	type configuration struct {
+		Parent struct {
+			Token string `yaml:"token" configkit:"secret"`
+		} `yaml:"parent"`
+		Child entry `yaml:"child"`
+	}
+	schema, issue := buildSchema(reflect.TypeFor[configuration](), testSecretBinding())
+	if issue != nil {
+		t.Fatal(issue)
+	}
+	inspection, _ := inheritanceInspectionForTest(t, parseCompositionDocument(t, "source-1", "parent: {token: TAGGED-CANARY}\nchild: {extends: parent}\n"))
+	inspection.secrets = classifySecretPaths(schema, nil, inspection.metadata, inspection.final, inspection.applications, inspection.input)
+	child, found := inspection.Explain(Keys("child", "token"))
+	if !found || !child.Effective.Redacted || strings.Contains(child.Effective.YAML, "TAGGED-CANARY") {
+		t.Fatalf("tagged source copy = %#v, %v", child, found)
+	}
+}
+
+func TestInheritedSecretReferencesResolveOnlyAfterExpansion(t *testing.T) {
+	type entry struct {
+		Credential Secret `yaml:"credential"`
+	}
+	type configuration struct {
+		Entries map[string]entry `yaml:"entries"`
+	}
+	schema, issue := buildSchema(reflect.TypeFor[configuration](), testSecretBinding())
+	if issue != nil {
+		t.Fatal(issue)
+	}
+	composed, diagnostics := composeDocuments(parseCompositionDocument(t, "source-1", "entries:\n  parent: {credential: INHERITED_SECRET_NAME}\n  child: {extends: parent}\n"))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	expanded, diagnostics := expandInheritanceWithOrigins(composed.root, composed.origins, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	lookupCount := 0
+	decoded, issues := decodeConfiguration(schema, reflect.Zero(reflect.TypeFor[configuration]()), expanded.root)
+	if len(issues) != 0 || lookupCount != 0 {
+		t.Fatalf("decoding and lookup order = %#v, %d", issues, lookupCount)
+	}
+	resolved := resolveSecretReferences(decoded.value, schema, expanded.root, func(name string) (string, bool) {
+		lookupCount++
+		if name != "INHERITED_SECRET_NAME" {
+			t.Fatalf("inherited reference name = %q", name)
+		}
+		return "RESOLVED-CREDENTIAL-CANARY", true
+	})
+	if len(resolved.diagnostics) != 0 || lookupCount != 1 ||
+		decoded.value.Interface().(configuration).Entries["child"].Credential.Value() != "RESOLVED-CREDENTIAL-CANARY" {
+		t.Fatalf("inherited reference resolution = %#v, %d", resolved.diagnostics, lookupCount)
+	}
+	inspection, _ := inheritanceInspectionFromComposition(t, composed)
+	inspection.secrets = classifySecretPaths(schema, nil, inspection.metadata, inspection.final, inspection.applications, inspection.input)
+	var rendered bytes.Buffer
+	if err := inspection.WriteEffective(&rendered); err != nil || strings.Contains(rendered.String(), "RESOLVED-CREDENTIAL-CANARY") {
+		t.Fatalf("reference rendering = %q, %v", rendered.String(), err)
+	}
+}

internal/engine/inheritance_test.go

@@ -0,0 +1,159 @@
+package engine
+
+import (
+	"reflect"
+	"strconv"
+	"strings"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestSiblingInheritanceResolvesReferencesAndKeepsInputs(t *testing.T) {
+	tests := []struct {
+		name, input string
+		path        Path
+		text        string
+		transfers   int
+		controls    int
+	}{
+		{
+			name: "forward reference", input: "child: {extends: parent, value: own}\nparent: {value: parent, kept: preserved}\n",
+			path: Keys("child", "kept"), text: "preserved", transfers: 1, controls: 1,
+		},
+		{
+			name: "backward reference", input: "parent: {value: parent, kept: preserved}\nchild: {extends: parent, value: own}\n",
+			path: Keys("child", "value"), text: "own", transfers: 1, controls: 1,
+		},
+		{
+			name: "chain and shared parent", input: "last: {extends: middle, enabled: false}\nfirst: {value: original}\nmiddle: {extends: first, extra: middle}\nother: {extends: first}\n",
+			path: Keys("last", "value"), text: "original", transfers: 3, controls: 3,
+		},
+		{
+			name: "exact empty sibling name", input: "child: {extends: \"\"}\n\"\": {value: empty}\n",
+			path: Keys("child", "value"), text: "empty", transfers: 1, controls: 1,
+		},
+		{
+			name: "exact unusual sibling name", input: "child: {extends: \"a/b~c\"}\n\"a/b~c\": {value: unusual}\n",
+			path: Keys("child", "value"), text: "unusual", transfers: 1, controls: 1,
+		},
+		{
+			name: "null cancellation", input: "child: {extends: null, value: own}\nparent: {value: parent}\n",
+			path: Keys("child", "value"), text: "own", controls: 1,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			input := parseCompositionDocument(t, "source-1", test.input)
+			before := document.Clone(input)
+			result, diagnostics := expandSiblingDefinitions(input, Path{})
+			if len(diagnostics) != 0 {
+				t.Fatalf("diagnostics = %#v", diagnostics)
+			}
+			if len(result.transfers) != test.transfers || len(result.controls) != test.controls {
+				t.Fatalf("evidence = %#v, %#v", result.transfers, result.controls)
+			}
+			node, found := documentAtPath(result.root, test.path)
+			if !found || node.Text != test.text {
+				t.Fatalf("%s = %#v, %v", test.path.String(), node, found)
+			}
+			if _, found := documentAtPath(result.root, Keys("child", inheritanceKey)); found {
+				t.Fatal("child directive remained in expanded payload")
+			}
+			if !reflect.DeepEqual(input, before) {
+				t.Fatal("resolver changed input")
+			}
+			node.Text = "changed"
+			if !reflect.DeepEqual(input, before) {
+				t.Fatal("expanded payload aliases input")
+			}
+		})
+	}
+}
+
+func TestSiblingInheritanceTracksTransferAndIndependentChildren(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", "first: {value: parent}\nsecond: {extends: first}\nthird: {extends: first}\n")
+	result, diagnostics := expandSiblingDefinitions(input, Keys("catalog"))
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	if len(result.transfers) != 2 || result.transfers[0].from.String() != "/catalog/first" ||
+		result.transfers[0].to.String() != "/catalog/second" ||
+		result.transfers[0].reference.String() != "/catalog/second/extends" ||
+		len(result.transfers[0].locations) != 1 || result.transfers[0].locations[0].SourceID != "source-1" {
+		t.Fatalf("transfers = %#v", result.transfers)
+	}
+	second, _ := documentAtPath(result.root, Keys("second", "value"))
+	third, _ := documentAtPath(result.root, Keys("third", "value"))
+	second.Text = "changed"
+	if third.Text != "parent" {
+		t.Fatal("siblings share mutable expanded values")
+	}
+}
+
+func TestSiblingInheritanceRejectsInvalidRelationships(t *testing.T) {
+	tests := []struct {
+		name, input, path string
+		code              Code
+		related           bool
+	}{
+		{"missing", "child: {extends: private-reference-canary}\n", "/child/extends", CodeInheritanceMissing, false},
+		{"nonmapping parent", "child: {extends: parent}\nparent: null\n", "/child/extends", CodeInheritanceInvalid, true},
+		{"boolean reference", "child: {extends: true}\n", "/child/extends", CodeInheritanceInvalid, false},
+		{"sequence reference", "child: {extends: [parent]}\n", "/child/extends", CodeInheritanceInvalid, false},
+		{"self reference", "child: {extends: child}\n", "/child/extends", CodeInheritanceCycle, true},
+		{"longer cycle", "a: {extends: b}\nb: {extends: c}\nc: {extends: a}\n", "/c/extends", CodeInheritanceCycle, true},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			input := parseCompositionDocument(t, "source-1", test.input)
+			result, diagnostics := expandSiblingDefinitions(input, Path{})
+			if result != nil || len(diagnostics) != 1 {
+				t.Fatalf("result and diagnostics = %#v, %#v", result, diagnostics)
+			}
+			problem := diagnostics[0]
+			if problem.Code != test.code || problem.Path.String() != test.path || problem.Primary == nil ||
+				problem.Primary.SourceID != "source-1" || problem.Primary.Line < 1 ||
+				(len(problem.Related) != 0) != test.related || strings.Contains(problem.Message, "private-reference-canary") {
+				t.Fatalf("diagnostic = %#v", problem)
+			}
+		})
+	}
+}
+
+func TestLoadDetectsInvalidInheritanceInsideSequence(t *testing.T) {
+	input := []byte(`items:
+  - child: {extends: true}
+`)
+	result, err := testLoad[map[string]any](Bytes("nested-inheritance.yml", input), Options[map[string]any]{})
+	if result != nil || err == nil {
+		t.Fatalf("result, err = %#v, %v; want invalid nested inheritance", result, err)
+	}
+	loadError, ok := err.(*LoadError)
+	if !ok {
+		t.Fatalf("error type = %T, want *LoadError", err)
+	}
+	diagnostics := loadError.Diagnostics()
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeInheritanceInvalid || diagnostics[0].Path.String() != "/items/0/child/extends" {
+		t.Fatalf("diagnostics = %#v", diagnostics)
+	}
+}
+
+func TestSiblingInheritanceDetectsDeepCycleWithoutRecursiveDependencyWalk(t *testing.T) {
+	const count = 2048
+	entries := make([]document.Entry, count)
+	for index := range entries {
+		location := document.Location{SourceID: "source-1", Line: index + 1, Column: 1}
+		parent := strconv.Itoa((index + 1) % count)
+		entries[index] = document.Entry{
+			Key: strconv.Itoa(index), Value: &document.Node{Kind: document.Mapping, Location: location,
+				Mapping: []document.Entry{{Key: inheritanceKey, Value: &document.Node{
+					Kind: document.String, Text: parent, Location: location,
+				}}}},
+		}
+	}
+	result, diagnostics := expandSiblingDefinitions(&document.Node{Kind: document.Mapping, Mapping: entries}, Path{})
+	if result != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeInheritanceCycle {
+		t.Fatalf("deep cycle result = %#v, diagnostics = %#v", result, diagnostics)
+	}
+}

internal/engine/inheritance_walk.go

@@ -0,0 +1,309 @@
+package engine
+
+import (
+	"cmp"
+	"slices"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+type inheritanceWalker struct {
+	nodes        int // logical non-directive nodes in the entire working tree
+	origins      map[string]Origin
+	limits       projectionLimits
+	retained     *retainedNodeCounter
+	transfers    []inheritanceTransfer
+	controls     []inheritanceControl
+	replacements []inheritanceReplacement
+	operations   []inheritanceOperation
+}
+
+// expandInheritanceTree resolves each destination scope after its enclosing
+// relationships. The loading lifecycle uses the origin-aware entry point.
+func expandInheritanceTree(input *document.Node, limits projectionLimits) (*inheritanceScope, []Diagnostic) {
+	return expandInheritanceWithOrigins(input, nil, limits)
+}
+
+func expandInheritanceWithOrigins(input *document.Node, inputOrigins map[string]Origin, limits projectionLimits) (*inheritanceScope, []Diagnostic) {
+	return expandInheritanceWithBudgets(input, inputOrigins, limits, historyNodeLimit)
+}
+
+func expandInheritanceWithBudgets(input *document.Node, inputOrigins map[string]Origin, limits projectionLimits, historyLimit int) (*inheritanceScope, []Diagnostic) {
+	if input == nil || input.Kind != document.Mapping {
+		return nil, []Diagnostic{{Code: CodeInheritanceInvalid, Severity: SeverityError,
+			Message: "inheritance requires a mapping document root"}}
+	}
+	root := document.Clone(input)
+	origins := cloneOriginIndex(inputOrigins)
+	if inputOrigins == nil {
+		origins = directInheritanceOrigins(input, Path{}, nil)
+	}
+	nodes, within := inheritanceLowerBoundCount(root, limits.nodes)
+	if !within {
+		return nil, []Diagnostic{inheritanceBudgetDiagnostic(Path{}, root.Location)}
+	}
+	walker := inheritanceWalker{
+		nodes: nodes, limits: limits, origins: origins,
+		retained: newRetainedNodeCounter(historyLimit),
+	}
+	if problem := walker.visit(&root, Path{}, true); problem != nil {
+		return nil, []Diagnostic{*problem}
+	}
+	if problem := inheritanceFinalBudget(root, limits); problem != nil {
+		return nil, []Diagnostic{*problem}
+	}
+	return &inheritanceScope{
+		root: root, origins: walker.origins, transfers: walker.transfers,
+		controls: walker.controls, replacements: walker.replacements, operations: walker.operations,
+	}, nil
+}
+
+func (w *inheritanceWalker) visit(node **document.Node, path Path, unscoped bool) *Diagnostic {
+	switch (*node).Kind {
+	case document.Mapping:
+		original := *node
+		if unscoped {
+			reference, found := inheritanceDirective(*node)
+			if found {
+				referencePath := path.Key(inheritanceKey)
+				location := publicLocation(reference.Value.Location)
+				if reference.Value.Kind != document.Null {
+					problem := inheritanceDiagnostic(CodeInheritanceInvalid, referencePath, location, nil,
+						"inheritance reference has no enclosing sibling scope")
+					return &problem
+				}
+				w.controls = append(w.controls, inheritanceControl{
+					path: referencePath, locations: []Location{location}, cancelled: true,
+				})
+				origin := directInheritanceOrigins(reference.Value, referencePath, w.origins)[referencePath.String()]
+				control := inheritanceControl{path: referencePath, locations: []Location{location}, cancelled: true}
+				w.operations = append(w.operations, inheritanceOperation{control: &control, controlOrigin: origin})
+				deleteOriginSubtree(w.origins, referencePath, reference.Value)
+				*node = withoutInheritanceDirective(*node)
+			}
+		}
+		if scopeHasInheritanceDefinitions(*node) {
+			// Only this scope can change the logical total. Reserve the nodes
+			// outside it so expansion checks the remaining budget before cloning.
+			before, _ := inheritanceLowerBoundCount(*node, -1)
+			limits := w.limits
+			if limits.nodes >= 0 {
+				limits.nodes -= w.nodes - before
+			}
+			resolved, diagnostics := expandSiblingDefinitionsWithOrigins(*node, path, limits, w.origins, w.retained)
+			if len(diagnostics) != 0 {
+				return &diagnostics[0]
+			}
+			*node = resolved.root
+			deleteOriginSubtree(w.origins, path, original)
+			for pointer, origin := range resolved.origins {
+				w.origins[pointer] = origin
+			}
+			w.transfers = append(w.transfers, resolved.transfers...)
+			w.controls = append(w.controls, resolved.controls...)
+			w.replacements = append(w.replacements, resolved.replacements...)
+			w.operations = append(w.operations, resolved.operations...)
+			after, _ := inheritanceLowerBoundCount(*node, -1)
+			w.nodes += after - before
+		}
+		// Operations retain immutable enclosing templates. Detach just this
+		// container before replacing descendants during destination resolution;
+		// untouched subtrees remain shared with the retained evidence.
+		working := **node
+		working.Mapping = slices.Clone(working.Mapping)
+		*node = &working
+		indices := make([]int, len((*node).Mapping))
+		for index := range indices {
+			indices[index] = index
+		}
+		slices.SortFunc(indices, func(left, right int) int {
+			return cmp.Compare((*node).Mapping[left].Key, (*node).Mapping[right].Key)
+		})
+		for _, index := range indices {
+			entry := &(*node).Mapping[index]
+			if problem := w.visit(&entry.Value, path.Key(entry.Key), false); problem != nil {
+				return problem
+			}
+		}
+	case document.Sequence:
+		working := **node
+		working.Sequence = slices.Clone(working.Sequence)
+		*node = &working
+		for index := range (*node).Sequence {
+			child := &(*node).Sequence[index]
+			if problem := w.visit(child, path.Index(index), (*child).Kind == document.Mapping); problem != nil {
+				return problem
+			}
+		}
+	}
+	return nil
+}
+
+func scopeHasInheritanceDefinitions(scope *document.Node) bool {
+	for _, entry := range scope.Mapping {
+		if entry.Value.Kind != document.Mapping {
+			continue
+		}
+		if _, found := inheritanceDirective(entry.Value); found {
+			return true
+		}
+	}
+	return false
+}
+
+// A directive may disappear during later destination resolution, so only
+// non-directive nodes form a safe early lower bound on final output size.
+func inheritanceLowerBoundWithin(root *document.Node, limit int) bool {
+	_, within := inheritanceLowerBoundCount(root, limit)
+	return within
+}
+
+func inheritanceLowerBoundCount(root *document.Node, limit int) (int, bool) {
+	if limit < 0 {
+		limit = int(^uint(0) >> 1)
+	}
+	stack := []*document.Node{root}
+	count := 0
+	for len(stack) != 0 {
+		node := stack[len(stack)-1]
+		stack = stack[:len(stack)-1]
+		if node == nil {
+			continue
+		}
+		count++
+		if count > limit {
+			return count, false
+		}
+		switch node.Kind {
+		case document.Mapping:
+			for _, entry := range node.Mapping {
+				if entry.Key == inheritanceKey {
+					continue
+				}
+				count++ // mapping key
+				stack = append(stack, entry.Value)
+			}
+		case document.Sequence:
+			stack = append(stack, node.Sequence...)
+		}
+		if count > limit {
+			return count, false
+		}
+	}
+	return count, true
+}
+
+// Count the materialized mapping shape before cloning a memoized parent.
+// Relationship fields are excluded because later destination resolution
+// consumes them; their omission makes this a safe lower bound.
+func inheritanceOverlayLowerBoundWithin(lower, upper *document.Node, limit int) bool {
+	if limit < 0 {
+		return false
+	}
+	index := new(mappingIndex)
+	count := 0
+	var visit func(*document.Node, *document.Node) bool
+	visit = func(base, incoming *document.Node) bool {
+		if base.Kind != document.Mapping || incoming.Kind != document.Mapping {
+			added, within := inheritanceLowerBoundCount(incoming, limit-count)
+			count += added
+			return within
+		}
+		count++ // mapping node
+		if count > limit {
+			return false
+		}
+		for _, entry := range base.Mapping {
+			if entry.Key == inheritanceKey {
+				continue
+			}
+			count++ // mapping key
+			if count > limit {
+				return false
+			}
+			position := index.find(incoming, entry.Key)
+			if position >= 0 {
+				if !visit(entry.Value, incoming.Mapping[position].Value) {
+					return false
+				}
+			} else {
+				added, within := inheritanceLowerBoundCount(entry.Value, limit-count)
+				count += added
+				if !within {
+					return false
+				}
+			}
+		}
+		for _, entry := range incoming.Mapping {
+			if entry.Key == inheritanceKey || index.find(base, entry.Key) >= 0 {
+				continue
+			}
+			count++ // mapping key
+			if count > limit {
+				return false
+			}
+			added, within := inheritanceLowerBoundCount(entry.Value, limit-count)
+			count += added
+			if !within || count > limit {
+				return false
+			}
+		}
+		return true
+	}
+	return visit(lower, upper)
+}
+
+func inheritanceFinalBudget(root *document.Node, limits projectionLimits) *Diagnostic {
+	type pending struct {
+		node *document.Node
+		path Path
+	}
+	stack := []pending{{node: root}}
+	count := 0
+	for len(stack) != 0 {
+		item := stack[len(stack)-1]
+		stack = stack[:len(stack)-1]
+		if len(item.path.segments) > limits.depth {
+			problem := inheritanceBudgetDiagnostic(item.path, item.node.Location)
+			return &problem
+		}
+		count++
+		if limits.nodes >= 0 && count > limits.nodes {
+			problem := inheritanceBudgetDiagnostic(item.path, item.node.Location)
+			return &problem
+		}
+		switch item.node.Kind {
+		case document.Mapping:
+			count += len(item.node.Mapping) // mapping keys
+			for index := len(item.node.Mapping) - 1; index >= 0; index-- {
+				entry := item.node.Mapping[index]
+				stack = append(stack, pending{node: entry.Value, path: item.path.Key(entry.Key)})
+			}
+		case document.Sequence:
+			for index := len(item.node.Sequence) - 1; index >= 0; index-- {
+				stack = append(stack, pending{node: item.node.Sequence[index], path: item.path.Index(index)})
+			}
+		}
+		if limits.nodes >= 0 && count > limits.nodes {
+			problem := inheritanceBudgetDiagnostic(item.path, item.node.Location)
+			return &problem
+		}
+	}
+	return nil
+}
+
+func inheritanceBudgetDiagnostic(path Path, location document.Location) Diagnostic {
+	primary := publicLocation(location)
+	return Diagnostic{
+		Code: CodeSchemaProjectionLimit, Severity: SeverityError, Path: path,
+		Message: "expanded configuration exceeds the projection limit", Primary: &primary,
+	}
+}
+
+func inheritanceHistoryBudgetDiagnostic(path Path, location document.Location) Diagnostic {
+	primary := publicLocation(location)
+	return Diagnostic{
+		Code: CodeExplainLimitExceeded, Severity: SeverityError, Path: path,
+		Message: "retained inheritance history exceeds the node limit", Primary: &primary,
+	}
+}

internal/engine/inheritance_walk_test.go

@@ -0,0 +1,219 @@
+package engine
+
+import (
+	"reflect"
+	"testing"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
+
+func TestNestedInheritanceUsesEachDestinationScope(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", `presets:
+  base:
+    settings:
+      primary: {width: 10}
+      secondary: {extends: primary}
+  custom:
+    extends: base
+    settings:
+      primary: {width: 20}
+`)
+	before := document.Clone(input)
+	result, diagnostics := expandInheritanceTree(input, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	for path, want := range map[string]string{
+		"/presets/base/settings/secondary/width":   "10",
+		"/presets/custom/settings/secondary/width": "20",
+	} {
+		parsed, _ := ParsePath(path)
+		node, found := documentAtPath(result.root, parsed)
+		if !found || node.Text != want {
+			t.Errorf("%s = %#v, found %v", path, node, found)
+		}
+	}
+	if len(result.transfers) != 3 || !reflect.DeepEqual(input, before) {
+		t.Fatalf("transfers = %#v; input changed = %v", result.transfers, !reflect.DeepEqual(input, before))
+	}
+	if node, found := documentAtPath(result.root, Keys("presets", "custom", "settings", "secondary", inheritanceKey)); found {
+		t.Fatalf("nested directive survived: %#v", node)
+	}
+}
+
+func TestNestedInheritanceSupportsChainsCancellationAndContainerReplacement(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", `catalog:
+  alpha:
+    settings:
+      first: {value: one}
+      second: {extends: first}
+  beta:
+    extends: alpha
+    settings:
+      first: {value: two}
+  gamma:
+    extends: beta
+    settings:
+      first: {value: three}
+  cancelled:
+    extends: alpha
+    settings:
+      second: {extends: null, value: own}
+  replaced:
+    extends: alpha
+    settings: none
+`)
+	result, diagnostics := expandInheritanceTree(input, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	checks := map[string]string{
+		"/catalog/alpha/settings/second/value":     "one",
+		"/catalog/beta/settings/second/value":      "two",
+		"/catalog/gamma/settings/second/value":     "three",
+		"/catalog/cancelled/settings/second/value": "own",
+		"/catalog/replaced/settings":               "none",
+	}
+	for pointer, want := range checks {
+		path, _ := ParsePath(pointer)
+		node, found := documentAtPath(result.root, path)
+		if !found || node.Text != want {
+			t.Errorf("%s = %#v, found %v", pointer, node, found)
+		}
+	}
+	if _, found := documentAtPath(result.root, Keys("catalog", "replaced", "settings", "second")); found {
+		t.Fatal("replaced container retained an active child")
+	}
+	if _, found := documentAtPath(result.root, Keys("catalog", "cancelled", "settings", "second", inheritanceKey)); found {
+		t.Fatal("cancelled nested directive survived")
+	}
+}
+
+func TestNestedInheritanceCanReplaceAnInheritedRelationship(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", `base:
+  settings:
+    first: {value: old}
+    other: {value: new}
+    selected: {extends: first}
+custom:
+  extends: base
+  settings:
+    selected: {extends: other}
+`)
+	result, diagnostics := expandInheritanceTree(input, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	base, found := documentAtPath(result.root, Keys("base", "settings", "selected", "value"))
+	if !found || base.Text != "old" {
+		t.Fatalf("base relationship = %#v, %v", base, found)
+	}
+	custom, found := documentAtPath(result.root, Keys("custom", "settings", "selected", "value"))
+	if !found || custom.Text != "new" {
+		t.Fatalf("replaced relationship = %#v, %v", custom, found)
+	}
+}
+
+func TestNestedInheritanceRejectsDestinationCycleAndInvalidPositions(t *testing.T) {
+	tests := []struct {
+		name, input, path string
+		code              Code
+	}{
+		{
+			name: "destination-only cycle", input: `base:
+  settings:
+    a: {value: one}
+    b: {extends: a}
+custom:
+  extends: base
+  settings:
+    a: {extends: b}
+`, path: "/custom/settings/b/extends", code: CodeInheritanceCycle,
+		},
+		{"root reference", "extends: anything\nentry: {value: one}\n", "/extends", CodeInheritanceInvalid},
+		{"list element reference", "items: [{extends: anything, value: one}]\n", "/items/0/extends", CodeInheritanceInvalid},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			input := parseCompositionDocument(t, "source-1", test.input)
+			result, diagnostics := expandInheritanceTree(input, defaultProjectionLimits())
+			if result != nil || len(diagnostics) != 1 || diagnostics[0].Code != test.code ||
+				diagnostics[0].Path.String() != test.path || diagnostics[0].Primary == nil {
+				t.Fatalf("result = %#v, diagnostics = %#v", result, diagnostics)
+			}
+		})
+	}
+}
+
+func TestNestedInheritanceResolvesScopesInsideSequences(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", `items:
+  - primary: {value: first}
+    secondary: {extends: primary}
+  - extends: null
+    zeta: {extends: alpha}
+    alpha: {value: second}
+`)
+	result, diagnostics := expandInheritanceTree(input, defaultProjectionLimits())
+	if len(diagnostics) != 0 {
+		t.Fatal(diagnostics)
+	}
+	first, found := documentAtPath(result.root, Keys("items").Index(0).Key("secondary").Key("value"))
+	if !found || first.Text != "first" {
+		t.Fatalf("first sequence scope = %#v, %v", first, found)
+	}
+	second, found := documentAtPath(result.root, Keys("items").Index(1).Key("zeta").Key("value"))
+	if !found || second.Text != "second" {
+		t.Fatalf("second sequence scope = %#v, %v", second, found)
+	}
+	if _, found := documentAtPath(result.root, Keys("items").Index(1).Key(inheritanceKey)); found {
+		t.Fatal("sequence-element cancellation survived")
+	}
+}
+
+func TestInheritanceExpansionHonorsOutputNodeAndDepthBudgets(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", "base: {value: one}\nchild: {extends: base}\n")
+	for _, test := range []struct {
+		name   string
+		limits projectionLimits
+		valid  bool
+	}{
+		{name: "at node boundary", limits: projectionLimits{nodes: 9, depth: 2}, valid: true},
+		{name: "beyond node boundary", limits: projectionLimits{nodes: 8, depth: 2}},
+		{name: "at depth boundary", limits: projectionLimits{nodes: 20, depth: 2}, valid: true},
+		{name: "beyond depth boundary", limits: projectionLimits{nodes: 20, depth: 1}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, diagnostics := expandInheritanceTree(input, test.limits)
+			if test.valid {
+				if result == nil || len(diagnostics) != 0 {
+					t.Fatalf("result = %#v, diagnostics = %#v", result, diagnostics)
+				}
+			} else if result != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeSchemaProjectionLimit {
+				t.Fatalf("result = %#v, diagnostics = %#v", result, diagnostics)
+			}
+		})
+	}
+}
+
+func TestInheritanceBudgetIncludesOtherScopesAndSequenceItems(t *testing.T) {
+	input := parseCompositionDocument(t, "source-1", `extends: null
+groups:
+  - a: {x: one}
+    b: {extends: a}
+  - a: {x: two}
+    b: {extends: a}
+unchanged: {x: three}
+`)
+	// Root (1), groups key/list (2), two expanded scopes (18), and
+	// unchanged key/mapping/key/value (4). Consumed controls cost nothing.
+	for _, limit := range []int{24, 25, 26} {
+		result, diagnostics := expandInheritanceTree(input, projectionLimits{nodes: limit, depth: 5})
+		if limit < 25 {
+			if result != nil || len(diagnostics) != 1 || diagnostics[0].Code != CodeSchemaProjectionLimit {
+				t.Fatalf("limit %d accepted or wrong error: %#v", limit, diagnostics)
+			}
+		} else if result == nil || len(diagnostics) != 0 {
+			t.Fatalf("limit %d rejected: %#v", limit, diagnostics)
+		}
+	}
+}

internal/engine/layered.go

@@ -0,0 +1,50 @@
+package engine
+
+import "os"
+
+// Input selects one source and its independent import policy.
+type Input struct {
+	Source     Source
+	Imports    bool
+	Filesystem *FilesystemContext
+}
+
+// Underlay contains strictly composed contributions at one precedence tier.
+type Underlay struct{ Inputs []Input }
+
+// LayeredOptions selects lower-priority tiers and final application rules.
+type LayeredOptions[T any] struct {
+	Definition        Definition[T]
+	Underlays         []Underlay
+	Overlay           string
+	OverlayFilesystem *FilesystemContext
+	Overrides         []Override
+}
+
+// LoadLayered loads a main input and lower-priority grouped inputs through one
+// final decode, resolution, and validation lifecycle.
+func LoadLayered[T any](main Input, secrets SecretBinding, supplied ...LayeredOptions[T]) (*Loaded[T], error) {
+	if len(supplied) > 1 {
+		return nil, (&diagnosticAccumulator{diagnostics: []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "LoadLayered accepts at most one options value")}}).loadError()
+	}
+	var options LayeredOptions[T]
+	if len(supplied) != 0 {
+		options = supplied[0]
+	}
+	request := loadRequest[T]{
+		main:       loadInput{source: main.Source, imports: main.Imports, filesystem: main.Filesystem},
+		definition: options.Definition, overlay: options.Overlay,
+		overlayFilesystem: options.OverlayFilesystem,
+		overrides:         append([]Override(nil), options.Overrides...),
+		underlays:         make([]loadGroup, len(options.Underlays)),
+	}
+	for index, underlay := range options.Underlays {
+		request.underlays[index].inputs = make([]loadInput, len(underlay.Inputs))
+		for inputIndex, input := range underlay.Inputs {
+			request.underlays[index].inputs[inputIndex] = loadInput{
+				source: input.Source, imports: input.Imports, filesystem: input.Filesystem,
+			}
+		}
+	}
+	return loadGroupedWithSecretLookup(request, secrets, os.LookupEnv)
+}

internal/engine/load.go

@@ -33,6 +33,7 @@ type Options[T any] struct {
 type FilesystemContext struct {
 	BaseDir string
 	Root    string
+	FS      *Filesystem
 }
 
 // Override describes one explicitly supplied application override. Value uses
@@ -77,13 +78,17 @@ type Loaded[T any] struct {
 type Inspection struct {
 	metadata         Metadata
 	sources          []SourceInfo
+	contexts         map[string]ResourceContext
 	diagnostics      []Diagnostic
 	baseline         *document.Node
 	input            *document.Node
 	inputOrigins     map[string]Origin
 	final            *document.Node
+	immutableLookup  *documentLookup
 	resolution       []resolutionSnapshot
+	resolutionReplay []resolutionSnapshot
 	applications     []historyApplication
+	inheritance      []inheritanceOperation
 	secrets          map[string]bool
 	secretReferences map[string]string
 }
@@ -104,6 +109,16 @@ func (r *Inspection) Sources() []SourceInfo {
 	return cloneSourceInfos(r.sources)
 }
 
+// SourceContext returns a passive filesystem view for an acquired source.
+// Unknown IDs and sources without a filesystem context return false.
+func (r *Inspection) SourceContext(id string) (ResourceContext, bool) {
+	if r == nil {
+		return ResourceContext{}, false
+	}
+	context, ok := r.contexts[id]
+	return context, ok
+}
+
 // Diagnostics returns detached nonfatal diagnostics from the load.
 func (r *Inspection) Diagnostics() []Diagnostic {
 	if r == nil {
@@ -120,14 +135,27 @@ func Load[T any](source Source, secrets SecretBinding, supplied ...Options[T]) (
 }
 
 func loadWithSecretLookup[T any](source Source, secrets SecretBinding, lookup secretLookup, supplied ...Options[T]) (*Loaded[T], error) {
-	accumulator := diagnosticAccumulator{}
 	options, diagnostics := selectOptions(source, supplied)
-	accumulator.add(diagnostics...)
 	if len(diagnostics) != 0 {
-		return nil, accumulator.loadError()
+		return nil, (&diagnosticAccumulator{diagnostics: diagnostics}).loadError()
+	}
+	request := loadRequest[T]{
+		main:       loadInput{source: source, imports: options.Imports, filesystem: options.Filesystem},
+		definition: options.Definition, overlay: options.Overlay,
+		overrides: options.Overrides, ordinary: true,
+	}
+	return executeLoadRequest(request, secrets, lookup)
 }
 
-	input, diagnostics := assembleBaseInput(source, options)
+func executeLoadRequest[T any](request loadRequest[T], secrets SecretBinding, lookup secretLookup) (*Loaded[T], error) {
+	accumulator := diagnosticAccumulator{}
+	accounting := &importAccounting{}
+	mainOptions := Options[T]{Definition: request.definition, Imports: request.main.imports, Filesystem: request.main.filesystem}
+	if request.ordinary {
+		mainOptions.Overlay = request.overlay
+	}
+
+	input, diagnostics := assembleBaseInputTracked(request.main.source, mainOptions, accounting, nil, nil)
 	accumulator.sources = input.sources
 	accumulator.add(diagnostics...)
 	if len(diagnostics) != 0 {
@@ -138,29 +166,68 @@ func loadWithSecretLookup[T any](source Source, secrets SecretBinding, lookup se
 		accumulator.add(diagnosticFromSchemaIssue(schemaIssue))
 		return nil, accumulator.loadError()
 	}
-	versionDiagnostics := checkVersion(options.Definition.Version, input.document)
+	versionDiagnostics := checkVersion(request.definition.Version, input.document)
 	accumulator.add(versionDiagnostics...)
 	if hasErrors(versionDiagnostics) {
 		return nil, accumulator.loadError()
 	}
-	input, diagnostics = applyInputLayers(source, options, input)
+	if request.ordinary {
+		input, diagnostics = applyInputLayers(request.main.source, Options[T]{Definition: request.definition, Overlay: request.overlay, Filesystem: request.main.filesystem, Overrides: request.overrides}, input)
+	} else {
+		input, diagnostics = assembleLayeredInput(request, input, accounting)
+	}
 	accumulator.sources = input.sources
 	accumulator.add(diagnostics...)
 	if len(diagnostics) != 0 {
 		return nil, accumulator.loadError()
 	}
+	if request.ordinary && input.parsed.source != nil {
+		baseSources := make([]SourceInfo, 0, len(input.sources))
+		for _, source := range input.sources {
+			if source.Layer == LayerBase {
+				baseSources = append(baseSources, source)
+			}
+		}
+		if len(baseSources) != 0 && input.contexts[baseSources[0].ID].FS == nil {
+			contexts, diagnostic := contextsForAcquiredSources(request.main.source, request.main.filesystem, input.parsed.source, baseSources)
+			if diagnostic != nil {
+				accumulator.add(*diagnostic)
+				return nil, accumulator.loadError()
+			}
+			mergeResourceContexts(&input.contexts, contexts)
+		}
+	}
+	if input.document != nil && containsInheritanceDirective(input.document) {
+		composition, problems := compositionForInput(input)
+		if len(problems) != 0 {
+			accumulator.add(problems...)
+			return nil, accumulator.loadError()
+		}
+		expanded, problems := expandInheritanceWithOrigins(composition.root, composition.origins, defaultProjectionLimits())
+		if len(problems) != 0 {
+			accumulator.add(problems...)
+			return nil, accumulator.loadError()
+		}
+		if problem := inheritanceVersionDiagnostic(request.definition.Version, expanded); problem != nil {
+			accumulator.add(*problem)
+			return nil, accumulator.loadError()
+		}
+		input.composition = composition
+		input.inheritance = expanded
+		input.document = expanded.root
+	}
 
-	decoded, diagnostics := decodeLoadInput(schema, options.Definition, input)
+	decoded, diagnostics := decodeLoadInput(schema, request.definition, input)
 	accumulator.add(diagnostics...)
 	if len(diagnostics) != 0 {
 		return nil, accumulator.loadError()
 	}
-	resolved, diagnostics := resolveLoadInput(schema, options.Definition, input, decoded)
+	resolved, diagnostics := resolveLoadInput(schema, request.definition, input, decoded)
 	accumulator.add(diagnostics...)
 	if hasErrors(diagnostics) {
 		return nil, accumulator.loadError()
 	}
-	if !historyNodesWithinLimit(decoded.baseline, decoded.final, resolved.final, resolved.applications, resolved.snapshots) {
+	if !historyNodesWithinLimit(decoded.baseline, decoded.final, resolved.final, resolved.applications, resolved.snapshots, input.inheritance) {
 		accumulator.add(Diagnostic{Code: CodeExplainLimitExceeded, Severity: SeverityError, Message: "retained configuration history exceeds the node limit"})
 		return nil, accumulator.loadError()
 	}
@@ -176,8 +243,8 @@ func loadWithSecretLookup[T any](source Source, secrets SecretBinding, lookup se
 	if hasErrors(secretResolution.diagnostics) {
 		return nil, accumulator.loadError()
 	}
-	if options.Definition.Validate != nil {
-		diagnostics = runValidation(&configuration, options.Definition.Validate, schema, resolved.final, input.document, secretResolution.lookups)
+	if request.definition.Validate != nil {
+		diagnostics = runValidation(&configuration, request.definition.Validate, schema, resolved.final, input.document, secretResolution.lookups)
 		accumulator.add(diagnostics...)
 		if hasErrors(diagnostics) {
 			return nil, accumulator.loadError()
@@ -186,5 +253,5 @@ func loadWithSecretLookup[T any](source Source, secrets SecretBinding, lookup se
 	resolved.configuration = configuration
 	// Publication detaches equal-valued validator replacements, preserving the
 	// validated secret state, then finalizes metadata and sensitivity.
-	return publishLoadResult(schema, options.Definition, resolved, secretResolution.references, &accumulator)
+	return publishLoadResult(schema, request.definition, resolved, secretResolution.references, &accumulator)
 }

internal/engine/load_input.go

@@ -1,24 +1,37 @@
 package engine
 
-import "gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+import (
+	"strconv"
+
+	"gitea.maximumdirect.net/eric/ConfigKit/internal/document"
+)
 
 // assembledInput is the source/document handoff across schema and version checks.
 type assembledInput struct {
 	parsed      parsedSource
 	document    *document.Node
 	composition *composedDocument
+	inheritance *inheritanceScope
 	sources     []SourceInfo
+	contexts    map[string]ResourceContext
 }
 
 func assembleBaseInput[T any](source Source, options Options[T]) (assembledInput, []Diagnostic) {
-	parsed, diagnostics := parseSource(source, "source-1", defaultParseLimits())
+	return assembleBaseInputTracked(source, options, &importAccounting{}, nil, nil)
+}
+
+func assembleBaseInputTracked[T any](source Source, options Options[T], accounting *importAccounting, visited map[resourceIdentity]importRoute, hostToken *namespaceToken) (assembledInput, []Diagnostic) {
+	limits := defaultImportLimits()
+	parseLimits := defaultParseLimits()
+	parseLimits.source.remaining = limits.bytes - accounting.bytes
+	parsed, diagnostics := parseSource(source, "source-"+strconv.Itoa(accounting.sources+1), parseLimits)
 	input := assembledInput{parsed: parsed, document: parsed.document}
 	if len(diagnostics) != 0 {
 		input.sources = sourceInfos(parsed.source)
 		return input, diagnostics
 	}
 	if options.Imports {
-		input.composition, input.sources, diagnostics = loadImportedDocuments(source, options.Filesystem, parsed, options.Definition.Version, defaultImportLimits())
+		input.composition, input.sources, diagnostics = loadImportedDocumentsTracked(source, options.Filesystem, parsed, options.Definition.Version, limits, accounting, visited, hostToken)
 		if len(diagnostics) != 0 {
 			return input, diagnostics
 		}
@@ -35,10 +48,44 @@ func assembleBaseInput[T any](source Source, options Options[T]) (assembledInput
 		if diagnostics := disabledDirectiveDiagnostics(input.document, Path{}); len(diagnostics) != 0 {
 			return input, diagnostics
 		}
+		if accounting.sources+1 > limits.sources || accounting.bytes+parsed.bytes > limits.bytes || accounting.nodes+countInputNodes(input.document) > limits.nodes {
+			return input, []Diagnostic{sourceDiagnostic(CodeSourceLimitExceeded, parsed.source.ID, "configuration exceeds the aggregate input limit")}
+		}
+		if diagnostic := registerAcquiredSource(source, options.Filesystem, parsed.source, visited, hostToken); diagnostic != nil {
+			return input, []Diagnostic{*diagnostic}
+		}
+		accounting.sources++
+		accounting.bytes += parsed.bytes
+		accounting.nodes += countInputNodes(input.document)
+	}
+	if !activeOverlay(options.Overlay) || options.Imports {
+		contexts, diagnostic := contextsForAcquiredSources(source, options.Filesystem, parsed.source, input.sources)
+		if diagnostic != nil {
+			return input, []Diagnostic{*diagnostic}
+		}
+		input.contexts = contexts
 	}
 	return input, nil
 }
 
+func registerAcquiredSource(source Source, context *FilesystemContext, info *SourceInfo, visited map[resourceIdentity]importRoute, hostToken *namespaceToken) *Diagnostic {
+	if visited == nil || info == nil || info.Path == "" {
+		return nil
+	}
+	identity := resourceIdentity{namespace: hostToken, path: info.Path}
+	if source.kind == sourceFilesystem {
+		identity.namespace = source.namespace.token
+	} else if context != nil && context.FS != nil {
+		identity.namespace = context.FS.namespace.token
+	}
+	if previous, repeated := visited[identity]; repeated {
+		value := importRepeatedDiagnostic(Location{SourceID: info.ID}, previous, importRoute{})
+		return &value
+	}
+	visited[identity] = importRoute{canonical: info.Path}
+	return nil
+}
+
 func applyInputLayers[T any](source Source, options Options[T], input assembledInput) (assembledInput, []Diagnostic) {
 	if !activeOverlay(options.Overlay) && len(options.Overrides) == 0 {
 		return input, nil
@@ -47,25 +94,31 @@ func applyInputLayers[T any](source Source, options Options[T], input assembledI
 	if len(diagnostics) != 0 {
 		return input, diagnostics
 	}
+	assembly := newAssemblyAccumulator(composition)
 	if activeOverlay(options.Overlay) {
-		composition, input.sources, diagnostics = loadSelectedOverlayDocument(
-			source, options.Filesystem, input.parsed, composition, input.sources,
+		input.sources, diagnostics = loadSelectedOverlayInto(
+			source, options.Filesystem, input.parsed, assembly, input.sources,
 			options.Overlay, options.Definition.Version, defaultImportLimits(),
 		)
 		if len(diagnostics) != 0 {
 			return input, diagnostics
 		}
+		contexts, diagnostic := contextsForAcquiredSources(source, options.Filesystem, input.parsed.source, input.sources[len(input.sources)-1:])
+		if diagnostic != nil {
+			return input, []Diagnostic{*diagnostic}
+		}
+		mergeResourceContexts(&input.contexts, contexts)
 	}
 	if len(options.Overrides) != 0 {
-		composition, input.sources, diagnostics = applyOverrides(
-			composition, input.sources, options.Overrides, options.Definition.Version, defaultImportLimits(),
+		input.sources, diagnostics = assembly.applyOverrides(
+			input.sources, options.Overrides, options.Definition.Version, defaultImportLimits(),
 		)
 		if len(diagnostics) != 0 {
 			return input, diagnostics
 		}
 	}
-	input.composition = composition
-	input.document = composition.root
+	input.composition = assembly.current
+	input.document = assembly.current.root
 	return input, nil
 }