Expose sequence placement controls in the public API

docs/config.md

@@ -67,13 +67,18 @@ ID `source-1`; enabled imports continue in depth-first order as `source-2` onwar
 contributes no source, and continues through defaults, decode, and projection. A
 missing file remains an error; it is never interpreted as no input.
 
-The current `Options` boundary accepts a defaults factory, `VersionRule`, resolver,
-semantic validator, explicit path and glob imports, one selected overlay, and
-filesystem context. It also accepts explicit CLI override records, static secret
-tags, and a dynamic path predicate. `NoSource` rejects imports, active overlays,
-and filesystem context as invalid combinations but accepts overrides. Input
-containing a reserved `configkit` mapping key is rejected while imports are
-disabled.
+`Options` accepts the application definition, ordinary-source `Destination` and
+`PlacementMode`, explicit path and glob imports, one selected overlay, filesystem
+context, explicit CLI overrides, and shared source-version controls. The definition
+may register strict sequence composition paths through
+`SequenceCompositions`. Its zero-valued placement mode places the source mapping
+at the destination; `sequence-item` explicitly wraps the mapping as one item at a
+non-root destination. Unknown modes, invalid destinations, and invalid or
+conflicting sequence registrations fail before source acquisition. Overlay and
+CLI coordinates remain rooted at the application root. `NoSource` rejects
+imports, active overlays, and filesystem context as invalid combinations but
+accepts overrides. Input containing a reserved `configkit` mapping key is rejected
+while imports are 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
@@ -176,12 +181,21 @@ follow the same strict composition within a tier and recursive precedence
 behavior described above. A selected overlay and CLI overrides remain rooted at
 the application root and are not moved beneath the main input's destination.
 
-`SourceInfo.Destination` reports the placement for each acquired input source,
-including imports. Placement does not change source IDs, acquisition identity,
-physical source locations, or passive resource contexts. Introduced ancestor
-mappings use the contributing document's root location as a source fallback;
-authored content retains its original source coordinates. Returned source
-records are detached copies.
+`SourceInfo.Destination` reports each acquired source's effective mapping or
+sequence-container path, including imports. `PlacementMode` reports how that
+source was placed; mapping is the zero value, and CLI/overlay sources report
+mapping unless item placement was applied. `DestinationUnresolved` is true only
+when an enclosing item index needed to name the source's destination has not yet
+been assigned. In that case `Destination` reports the nearest resolved enclosing
+collection container and omits the unresolved local suffix. A source's own item
+placement still has an exact destination at its sequence container. Completed
+loads project nested source destinations through their owning items. Callback and
+failed-load snapshots preserve the evidence available at that point; a fallback
+snapshot remains unresolved, while a later exact snapshot is false. Returned
+source records and paths are detached. Placement does not change source IDs,
+acquisition identity, physical source locations, or passive resource contexts.
+Introduced ancestor mappings use the contributing document's root location as a
+source fallback; authored content retains its original source coordinates.
 
 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,
@@ -744,12 +758,39 @@ Application defaults do not participate in this operation.
 | Mapping | Mapping | Recurse by mapping key |
 | Any other pair of present nodes | Any other pair of present nodes | `compose.conflict` |
 
-Every non-mapping overlap is a conflict, even when values are equal. Nulls and
-sequences are present atomic values, so they also conflict when repeated; sequences
-are never joined by index. Disjoint entries under shared mappings compose, making
+Every non-mapping overlap is a conflict, even when values are equal, except for
+sequence/sequence contributions at a registered sequence-composition path. Nulls
+remain atomic conflicts. Unregistered sequences conflict when repeated and are
+never joined by index. Disjoint entries under shared mappings compose, making
 stable keyed mappings the natural representation for independently contributed
 entries.
 
+`Definition.SequenceCompositions` registers literal mapping paths and one literal
+`IdentityField` for sequences that should compose independently authored records.
+Registration paths follow input-destination key rules: parsed numeric tokens are
+literal mapping keys, explicit index segments and reserved `configkit` or
+mapping-valued `extends` segments are invalid, and the root registration is
+harmless and unused. The identity field is one literal string key; neither the
+field name nor identity strings are normalized.
+At each registered path, every actual sequence contribution in a strict tier is
+validated, including a singleton contribution: each record must be a mapping with
+a present string identity field. Distinct exact identity strings append in source
+and list order; duplicate identities conflict even if the records are otherwise
+equal or disjoint. Empty identity strings are valid. Malformed records produce
+`compose.invalid`; duplicate identities produce `compose.conflict`, with the
+sequence container and source locations identifying a duplicate without exposing
+identity values. Missing paths and non-sequence values do not fail by themselves.
+Identical registrations coalesce; conflicting registrations at one path and
+invalid registration paths produce `options.invalid`. Unused and root registrations
+are allowed and have no effect.
+
+Registration changes strict composition only. Each underlay group and the main
+tier has independent duplicate-identity state. Precedence assembly is unchanged:
+a present higher-priority sequence replaces the lower sequence as one value,
+including at a registered path. Overlays, CLI overrides, defaults, inheritance,
+and application resolution do not run the registered identity check. The accepted
+rationale is recorded in [ADR 0014](adr/0014-sequence-placement-and-composition.md).
+
 An empty mapping contributes no child definitions. Two empty mappings compose, and
 an empty mapping neither clears nor conflicts with another mapping's children.
 Shared mapping containers retain all contributing value locations as their composite
@@ -889,8 +930,9 @@ Each path or glob declaration may include `placement: mapping` or
 including every match of a glob; placement is not inherited. Sequence-item
 placement requires a nonempty effective destination and wraps the imported
 mapping as one element of a sequence there. It does not split source sequences
-or enable sequence accumulation: overlapping sequences still follow the strict
-composition rules above.
+or infer identity fields. Repeated sequences still conflict unless their path is
+registered through `Definition.SequenceCompositions`; registration does not
+change placement or precedence behavior.
 
 An item import first opens a local mapping scope. Its body and ordinary helper
 imports compose within that record. An omitted helper destination or