Move option source construction into the internal engine

docs/roadmap/implementation.md

@@ -229,6 +229,17 @@ capacity managers, catalogs, or runners. Public options still have their existin
 names, signatures, ordering, capture, and error behavior. No transitional state
 mapping or duplicate source-assembly path remains.
 
+### Completion
+
+- [x] Wrapped the root option state around internal engine options with private
+  construction state.
+- [x] Moved filesystem and file source setup, source replacement, profile
+  repository construction, validator construction, and backend accumulation
+  into internal option methods.
+- [x] Kept ordered public option application, boundary conversion, collaborator
+  adaptation, and error translation in the root facade.
+- [x] Updated current architecture and internal ownership documentation.
+
 ## Stage 5: Guard The Public Boundary And Dependency Direction
 
 Purpose: protect the intended architecture without freezing private layout.