Move engine composition into the internal engine package

docs/roadmap/implementation.md

@@ -185,6 +185,15 @@ Acceptance: all public operations route through one internal engine and the
 existing runner; maintained catalogs and component wiring are internally owned.
 Prepared-execution behavior and public error contracts remain unchanged.
 
+### Completion
+
+- [x] Added `internal/engine` as the owner of engine assembly and runner
+  delegation.
+- [x] Kept the public facade's option application, conversions, nil checks,
+  prepared-execution wrapper, and error translation at the root.
+- [x] Updated architecture, component, source, and runner documentation for
+  the implemented ownership split.
+
 ## Stage 4: Move Option State And Source Construction Inward
 
 Purpose: finish the ownership split without changing the public option API.