Test typed sources through configuration composition

docs/roadmap/implementation.md

@@ -494,6 +494,10 @@ the helper itself neither accesses files nor resolves inheritance.
 
 ## Stage 10: Verify Typed Sources Through Real Composition
 
+**Completed:** Real engine composition tests cover inherited backend and prompt
+records, layered completion, snapshots, explicit values, relative inherited file
+content using ConfigKit's filesystem context, and deferred final validation.
+
 **Scope:** Focused source-helper contract tests, not a second ConfigKit suite.
 
 1. Extend existing backend-source tests with a real inherited child, preserving