Add snapshotted mapping configuration sources

docs/roadmap/implementation.md

@@ -336,6 +336,16 @@ their contracts. Run the everyday checker, including public-boundary tests.
 
 ## Stage 5: Add snapshotted mapping sources
 
+**Completed 2026-09-29.** `Mapping` now snapshots supported built-in values at
+construction, retaining safe deferred diagnostics for invalid roots, values,
+cycles, nonfinite floats, and limits. A shared private converter preserves CLI
+override behavior through a separate adapter. Mapping inputs participate in
+ordinary directives, imports, version checks, decoding, and inspection with
+their own source kind and synthetic locations. Public tests cover YAML
+equivalence, reuse after caller mutation, null/empty/zero distinctions, exact
+integers, explicit filesystem context, and safe failures. The everyday checker
+and engine race suite passed.
+
 1. Extract reusable value conversion from `override_value.go`; keep CLI-specific
    path, label, and diagnostic handling in its adapter.
 2. Implement `Mapping` at root and engine boundaries with constructor-time