Add an implementation plan and roadmap for Step 3 of the migration plan

docs/roadmap/migration.md

@@ -125,6 +125,9 @@ is accepted and records the required ownership and coordination decisions.
 Strengthen or add contract-focused tests where needed so extraction can be
 verified without relying on package placement.
 
+The accepted implementation scope and intended completion state are in the
+[Step 3 framework-characterization roadmap](step3.md).
+
 Preserve coverage of:
 
 - `Prepare` and `Run` behavior;