Add selected profile overlays

docs/roadmap/implementation.md

@@ -2,7 +2,7 @@
 
 ## Status and ownership
 
-Status: implementation in progress; Stages 1–9 are complete and later stages are
+Status: implementation in progress; Stages 1–13 are complete and later stages are
 pending.
 
 Audience: a `gpt-5.6-terra` coding agent implementing one stage per prompt, in
@@ -982,7 +982,31 @@ Acceptance:
 
 ## Stage 13 — Implement profile overlays
 
-Status: pending.
+Status: complete.
+
+Completion evidence (2026-09-06):
+
+- Added a private non-aliasing recursive overlay that merges mappings by key and
+  atomically replaces scalars, sequences, empty sequences, nulls, and cross-kind
+  subtrees. Replacement deletes stale descendant origins and records every displaced
+  definition plus the parent operation for later history.
+- Added exact selected-profile loading from `profiles/<name>.yml`, with constrained
+  names, inactive empty/`default` selection, canonical root containment, base-graph
+  duplicate rejection, deterministic profile source ownership, and shared aggregate
+  source/byte/node limits. No path selection, extension fallback, inheritance, or
+  implicit default file is supported.
+- Applied profiles only after base import composition and version checking and before
+  defaults and strict decoding. Profiles reject `_configkit` at any depth, direct
+  version contributions, and scalar/null replacements of version ancestors; base
+  parse and version failures therefore cannot be masked.
+- Tests cover recursive siblings, scalar/full/empty-sequence/null/cross-kind
+  replacement, lower defaults, profile source and origin attribution, removed-child
+  and stale-index history, selection errors, fixed extension, directives, version
+  placement, base aliases, parse ordering, filesystem context, symlink escape, input
+  budgets, and non-aliasing. The
+  [current profile contract](../config.md#selected-profiles) owns the behavior.
+  Verified `go test ./...`, `go test -race ./...`, `go vet ./...`, `go build ./...`,
+  and `git diff --check` with Go 1.26.7 on linux/amd64.
 
 Read: feature [profiles](initial.md#profiles), composition/overlay table.