Build reusable schema descriptors

docs/roadmap/implementation.md

@@ -2,7 +2,7 @@
 
 ## Status and ownership
 
-Status: implementation in progress; Stages 1–3 are complete and later stages are
+Status: implementation in progress; Stages 1–4 are complete and later stages are
 pending.
 
 Audience: a `gpt-5.6-terra` coding agent implementing one stage per prompt, in
@@ -463,7 +463,27 @@ Acceptance:
 
 ## Stage 4 — Build the reusable schema binding graph
 
-Status: pending.
+Status: complete.
+
+Completion evidence (2026-09-06):
+
+- Implemented a per-operation private descriptor graph in [`schema.go`](../../schema.go)
+  for structural roots, ordinary/named scalars, duration and paired text scalars,
+  pointers, structs, string-keyed maps, slices, arrays, dynamic values, and recursive
+  definitions. No global cache or application method execution is involved.
+- Implemented declared-field YAML binding, ignored/private fields, explicit struct/
+  pointer/map inlining, ambiguity and tag validation, one inline map, secret-tag
+  retention, and schema-aware key/index path traversal. Runtime default collisions,
+  cyclic values, cloning, projection, and decoding remain assigned to later stages.
+- Added the [current schema contract](../config.md#application-schema-binding) and
+  updated the component inventory and target-roadmap ownership links.
+- Focused tests cover nested and inline conflicts, ignored/private fields, maps of
+  structs, named scalars, pointer text method sets, missing codec pairs, root and
+  unsupported-kind rejection, recursive descriptors, side-effect freedom, dynamic
+  and inline-map paths, parsed numeric indices, and array bounds. Verified
+  `go test ./...`, `go vet ./...`, `go build ./...`, `go mod tidy -diff`, and
+  `git diff --check` with Go 1.26.7 on linux/amd64. No deviations were required;
+  typed value operations remain later work.
 
 Read: feature [typed decoding](initial.md#typed-decoding) and
 [type/projection contract](initial.md#supported-types-and-projection).