Add selected profile overlays

docs/internal/extraction.md

@@ -155,6 +155,17 @@ Assigned to selected-profile overlay:
   declared defaults, inheritance, arbitrary profile paths, or validating every
   unselected overlay.
 
+Implementation record (2026-09-06): `overlayDocuments`, `mergeOverlayMappings`,
+`loadProfileDocument`, and `profileDirectiveDiagnostics` adapt the assigned merge,
+selection, loading, and ownership mechanics. `TestOverlayDocumentsRecursesMappingsAndReplacesAtomicSubtrees`,
+`TestLoadAppliesSelectedProfileAfterBaseAndBeforeDefaults`, profile selection/error,
+directive/version, containment, and budget cases adapt the assigned tests. ConfigKit
+uses exactly one fixed selected `.yml` source, permits null and cross-kind replacement,
+retains displaced descendant history, rejects any profile directive or version-path
+contribution, and counts the profile in aggregate budgets. Narratio's declared
+default, multi-overlay validation, inheritance-like application conventions, and
+null/kind restrictions were not retained.
+
 ### Effective inspection and provenance
 
 Assigned to layered metadata, redaction, rendering, and explanation:

docs/internal/overview.md

@@ -13,6 +13,8 @@ module and package documentation boundary. Its implemented public primitives are
 - a private strict composer in `compose.go` that joins only disjoint document-tree
   definitions, retains ordered claims and composite mapping origins, reports every
   conflicting location, and never aliases an input tree;
+- fixed selected-profile loading in `profile.go` and recursive, non-aliasing overlay
+  with displacement records in `overlay.go`;
 - a per-operation private reflection descriptor graph in `schema.go` for supported
   types, YAML field binding, inlining, secret tags, recursion, and semantic paths;
 - descriptor-driven canonical projection, detached typed cloning, cycle/limit
@@ -31,7 +33,7 @@ module and package documentation boundary. Its implemented public primitives are
   `validation.go`, completing the public callback lifecycle after composed input.
 
 The [configuration contract](../config.md) owns implemented input, schema, decoding,
-loading lifecycle, path imports, composition, version checking, recorded resolution,
+loading lifecycle, path imports, composition, profile overlay, version checking, recorded resolution,
 semantic validation, metadata, and ownership behavior.
 
 The source-extraction inventory records the application foundations assigned to