Implement strict document composition

docs/roadmap/implementation.md

@@ -802,7 +802,27 @@ Acceptance:
 
 ## Stage 10 — Implement strict document composition
 
-Status: pending.
+Status: complete.
+
+Completion evidence (2026-09-06):
+
+- Adapted Narratio's additive claim validation and recursive mapping join into the
+  private `composeDocuments` boundary over ConfigKit-owned document trees. Present
+  equal or unequal scalars, nulls, sequences, mapping/value kind changes, and nested
+  duplicate entries return `compose.conflict`; mapping pairs alone recurse.
+- Changed the extracted empty-map behavior to ConfigKit's operation table: empty
+  mappings add no children, compose with other mappings, and retain their container
+  location in composite base origins. Application defaults never enter the composer.
+- Preserved source/declaration traversal in detached ordered claim records and used
+  it only for assembled order and exact primary/related diagnostics. Conflicts across
+  three or more inputs report every claimant, while shared containers aggregate all
+  contributing locations instead of selecting an arbitrary owner.
+- Cloned every retained branch and verified mutations of a composed result cannot
+  affect any input. Focused tests cover disjoint keyed joins, operation categories,
+  ancestor and nested conflicts, equal values, nulls, sequences, empty mappings,
+  exact locations, all claimants, invalid boundaries, and nonmutation. The
+  [current composition contract](../config.md#strict-document-composition) owns the
+  implemented semantics; public import wiring remains deferred to subsequent work.
 
 Read: feature [composition table](initial.md#composition-and-overlay).
 

docs/roadmap/initial.md

@@ -450,28 +450,24 @@ origin is a resolution operation, with input origins retained separately.
 
 ## Composition and overlay
 
-Composition combines the main file and explicitly imported fragments as peers.
-Repeated semantic definitions are errors, even when their values are equal.
-Both conflicting locations must be reported. Prefer mappings keyed by stable
-identifiers when entries need to be contributed independently.
+Strict base composition is implemented and now owned by the
+[current configuration contract](../config.md#strict-document-composition).
+Intentional profile overlay remains target behavior. Its exhaustive operation table
+is below; “value” includes scalar, sequence, mapping, and null, while absent means no
+node rather than null.
 
-The following table is exhaustive by node category. “Value” includes scalar,
-sequence, mapping, and null. Absent means no node, not null.
+| Existing/lower node | Incoming/higher node | Overlay |
+| --- | --- | --- |
+| Absent | Absent | Absent |
+| Absent | Value | Incoming |
+| Value | Absent | Existing |
+| Mapping | Mapping | Recurse by key |
+| Any other pair of present nodes | Any other pair of present nodes | Replace entire existing node |
 
-| Existing/lower node | Incoming/higher node | Strict composition | Overlay |
-| --- | --- | --- | --- |
-| Absent | Absent | Absent | Absent |
-| Absent | Value | Incoming | Incoming |
-| Value | Absent | Existing | Existing |
-| Mapping | Mapping | Recurse by key | Recurse by key |
-| Any other pair of present nodes | Any other pair of present nodes | Collision | Replace entire existing node |
-
-Two empty mappings compose successfully. An empty mapping contributes no child
-changes to an existing mapping; it does not clear that mapping. A mapping can
-replace a lower scalar, sequence, or null during overlay, subject to the final
-schema. Sequences are atomic: two definitions collide in composition and the
-higher sequence replaces the entire lower sequence in an overlay. `[]` clears a
-sequence. There is no append, union, index merge, or mapping-clear operator.
+A mapping can replace a lower scalar, sequence, or null during overlay, subject to
+the final schema. Sequences are atomic and the higher sequence replaces the entire
+lower sequence in an overlay. `[]` clears a sequence. There is no append, union,
+index merge, or mapping-clear operator.
 
 Replacing a parent must not leave effective metadata for removed children.
 Sequence histories describe whole-sequence replacement; index origins refer to