Extend collection inheritance architecture to identified sequences

docs/adr/0009-sibling-inheritance.md

@@ -3,8 +3,9 @@
 ## Status
 
 Partially superseded by [ADR 0017](0017-collection-sequence-policy.md) for
-sequence combination and [ADR 0018](0018-cli-after-structural-resolution.md) for
-CLI-before-inheritance ordering. The remaining inheritance scope, ownership, and
+sequence combination and the addition of identified-sequence sibling scopes,
+and [ADR 0018](0018-cli-after-structural-resolution.md) for CLI-before-inheritance
+ordering. Existing mapping-sibling scopes and the remaining ownership and
 inspection decisions remain accepted.
 
 The [configuration contract](../config.md#sibling-inheritance) owns implemented

docs/adr/0017-collection-sequence-policy.md

@@ -7,8 +7,9 @@ Accepted.
 This supersedes the decisions about who declares identity and how
 sequences combine in [ADR 0014](0014-sequence-placement-and-composition.md),
 and the rule that inheritance always replaces sequences in
-[ADR 0009](0009-sibling-inheritance.md). The existing source-placement and
-inheritance-scope rules continue to apply. It also supersedes the whole-sequence
+[ADR 0009](0009-sibling-inheritance.md). It also extends that ADR's sibling
+scopes to identified sequences. Existing mapping-sibling scopes and source
+placement continue to apply. It supersedes the whole-sequence
 precedence replacement rule in the proposed
 [ADR 0007](0007-source-backed-underlays.md).
 
@@ -31,6 +32,9 @@ also need a way to require unique identities in the final loaded configuration.
 These rules must work consistently for imported records, inline sequences,
 overlays, and inheritance.
 
+Records in a sequence may also share settings. Authors need to reuse those
+settings through inheritance while keeping each record's own identity.
+
 ## Decision
 
 ConfigKit manages identity and merge rules for each collection during loading.
@@ -52,6 +56,20 @@ identity (`upsert`). Source layering and sibling inheritance use the same
 operations. For inheritance, the rules at the inheriting destination determine
 which operation applies.
 
+An identified sequence is also a sibling scope for inheritance. A record may
+inherit from one other record in the same sequence, selected by its complete
+identity. Each record supplies its own identity before inheritance. The child
+keeps that identity and combines its settings with the parent's other fields,
+using the existing mapping and collection rules. Parents remain ordinary records
+subject to normal validation.
+
+Resolve these relationships after source assembly, using the same dependency
+ordering as mapping-sibling inheritance. Resolve nested relationships at each
+inheriting destination. Preserve the assembled sequence order. Whole-record
+upsert keeps its existing meaning; inheritance then uses the records that remain.
+Typed defaults, CLI overrides, and application resolution follow the lifecycle
+in [ADR 0018](0018-cli-after-structural-resolution.md).
+
 An explicitly selected operation remains in effect at its destination until a
 higher layer changes it. When no explicit operation applies, use upsert if an
 identity definition is available; otherwise replace the whole sequence. Authors
@@ -94,6 +112,10 @@ chooses how to store and track collection rules internally.
 - **Merge individual fields of records with matching identities.** Allows partial
   record updates, but adds rules for field conflicts and retained values.
   Replacing the whole record makes the update's scope clear.
+- **Keep inheritance limited to mapping siblings.** Keeps parent lookup simpler,
+  but authors of sequence records must duplicate shared settings or applications
+  must implement inheritance themselves. Collection identity provides a way to
+  select a parent within the existing loading process.
 - **Check every YAML identity again at the end of loading.** Gives YAML declarations
   authority over values changed by defaults, CLI assignments, and application
   resolution. This extends the lifetime of those declarations; application
@@ -112,6 +134,10 @@ which declarations still apply as records are replaced, copied, or moved to new
 positions. Lower-tier identity must be available before combining higher-tier
 contributions.
 
+Record inheritance also requires parent lookup by identity and checking for
+missing parents and cycles. Copied settings retain the same source, history,
+and sensitivity information as mapping-sibling inheritance.
+
 The configuration contract will define declaration syntax, identity comparison,
 omission and empty-value behavior, and diagnostics when the feature is
 implemented.

docs/policy/architecture.md

@@ -260,12 +260,22 @@ Consistent combination semantics do not require identical acquisition or lifetim
 behavior from every source kind.
 
 Built-in sibling `extends` relationships resolve after source precedence in the
-parser-independent document representation. Preserve their existing sibling
-scopes and resolve enclosing relationships before nested destination-local
-relationships. Mappings combine recursively; sequences use the inheriting
-collection's policy. Inheritance remains built-in and creates no acquired source.
+parser-independent document representation. Preserve existing mapping-sibling
+scopes and allow an identified sequence to provide a sibling scope for its
+records. A record may inherit from one parent in that sequence, selected by
+complete identity. The child supplies and keeps its own identity while inheriting
+the parent's other settings. Parents remain ordinary records subject to normal
+validation.
+
+Use the shared dependency ordering and preserve assembled sequence order.
+Resolve enclosing relationships before nested destination-local relationships.
+Mappings combine recursively; sequences use the inheriting collection's policy.
+Whole-record upsert replaces records during combination; explicit inheritance
+uses the records that remain. Inheritance remains built-in and creates no
+acquired source.
 [ADR 0009](../adr/0009-sibling-inheritance.md) records the
-continuing scope and ownership decisions, with sequence combination revised by
+original scope and continuing ownership decisions, with sequence combination
+and identified-sequence inheritance defined by
 [ADR 0017](../adr/0017-collection-sequence-policy.md) and CLI ordering by
 [ADR 0018](../adr/0018-cli-after-structural-resolution.md).