Cover shared debug binding composition

docs/roadmap/implementation.md

@@ -1,6 +1,6 @@
 # Shared debug binding implementation plan
 
-Status: in progress; Stages 1–3 complete.
+Status: in progress; Stages 1–4 complete.
 Baseline: `6727d22ce6e1ac668b56a3e6d8fa5f7a06e54d88`.
 Execution audience: Luna coding agent.
 
@@ -185,6 +185,12 @@ parse, without changing unrelated input indexes, grammar, or failure handling.
 
 ## Stage 4 — Preserve composition, defaults, and alias behavior
 
+**Complete.** Existing capture and rename machinery retains the boolean
+identity and requiredness without production changes. Focused public tests
+cover true defaults, explicit false, direct and captured requiredness, both
+application methods, renaming in both directions, and an alias to the local
+short spelling; capture and alias tests pass offline with Go 1.27.0.
+
 Primary files: [flag_values.go](../../flag_values.go),
 [binding.go](../../binding.go), [flag_aliases.go](../../flag_aliases.go).