Add internal engine relocation candidate

docs/internal/overview.md

@@ -3,6 +3,10 @@
 ConfigKit exposes the root `configkit` package and keeps its document and YAML
 representations private. The implemented components and their owners are:
 
+- a temporary `internal/engine` relocation candidate containing a mechanical
+  copy of the root loading, decoding, resolution, provenance, inspection, and
+  rendering mechanisms with their behavior tests; it imports no root code and
+  is not yet used by the root package or consumers;
 - immutable semantic `Path` construction and strict JSON Pointer parsing;
 - detached effective presence/origin and chronological history queries in
   `metadata.go`, with provenance and diagnostic record types;

docs/roadmap/implementation.md

@@ -233,6 +233,26 @@ no root code, existing internal/parser boundaries remain intact, and root consum
 still execute the unchanged original implementation. This stage is deliberately
 mechanical; do not mix ownership redesign into the relocation diff.
 
+### Completion
+
+Status: complete (2026-09-12).
+
+Copied the 39 root production files from the Stage 1 baseline `f81def9` into
+`internal/engine`, excluding only root package orientation in `doc.go`, and
+changed package declarations mechanically. Copied the 29 applicable root
+behavior and fuzz tests. Repository documentation, release-script and
+import-boundary checks, root external-package/API tests, and maintained consumer
+integration tests remain at their existing owners; root has not been changed to
+delegate to the candidate. No root testdata fixtures exist to copy.
+
+`docs/internal/overview.md` now labels the candidate as temporary and unused.
+`go list -deps` confirms that the engine has no dependency on the root module
+package; its existing dependencies remain the private document and YAML
+boundaries. Linux validation with Go 1.26.7 passed `go test ./internal/engine`
+and `./scripts/check.sh`. There were no behavioral deviations or package-renamed
+diagnostics. The next handoff replaces the copied engine's concrete public
+secret assumption with an explicit per-operation exact-type binding.
+
 ## Stage 3 — Decouple reflection from the public secret type
 
 Status: not started.