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).