Add overlay migration regression coverage

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Configuration Overlay Rename Implementation Plan
 
-Status: Stage 1 complete. Execute Stage 2.
+Status: complete.
 
 ## Scope and operating instructions
 
@@ -188,3 +188,23 @@ Report the stage commit, validation results, any skipped platform-specific cases
 and the API/filesystem migration summary. The implementation is complete only
 when both stages pass and the roadmap's target surface and preserved behavior
 are accounted for. There are no unresolved product questions in this plan.
+
+### Completion
+
+Completed 2026-09-23. Added regression coverage that accepts only
+`overlays/<name>.yml` for selection, rejects a legacy-only selected file with
+`CodeOverlayMissing`, and ignores a legacy sibling when the new selected file is
+present. Explicit imports from a directory named `profiles` remain base-layer
+imports. The maintained consumer confirms `ModelProfile` is unchanged while its
+selected source reports `overlays/testing.yml` and `LayerOverlay`; public
+layer/code strings and rendered explanation values are covered.
+
+Validation passed with `./scripts/check.sh` and `GOPROXY=off go test -race
+./internal/engine` on Go 1.26.8/linux. No additional platform-specific validation
+was required; existing symlink coverage remains conditional on Linux or macOS.
+The API audit found no legacy Go declarations, and the remaining `profile`
+references are intentionally limited to application-domain fields, upstream or
+historical records, migration guidance, and explicit legacy-path regressions. The
+feature roadmap is now a completion summary; v0.4.0 release preparation must still
+describe the Go API, selected-file directory, provenance layer, and diagnostic-code
+breaks.