Plan the multi-module implementation sprint

docs/roadmap/registration-backend.md

@@ -3,7 +3,8 @@
 Status: finalized internal design; no implementation has begun. Runnable-command
 children, scope-error attachment, and the unused-binding validation resolution are
 incorporated into the [registration API](registration-api.md). Both composition
-questions below are resolved; the staged implementation plan remains to be written.
+questions below are resolved; the [staged implementation plan](implementation.md)
+owns the ordered work and completion evidence.
 
 ## Purpose and authority