docs: mark Seriatim role contract stage complete

docs/roadmap/implementation.md

@@ -98,6 +98,8 @@ overrides: `GOWORK=off`, `GOTOOLCHAIN=local`, and `CGO_ENABLED=0`.
 
 ## Stage 1: Establish the Role Contract and Static Validation
 
+Completed: `ac35c1f` (`feat: define Seriatim role contract`)
+
 ### Objective
 
 Create a conventional role skeleton with a fully specified variable contract,
@@ -493,6 +495,4 @@ out-of-band execution. Completion requires subsequent review.
 
 ## Progress Notes
 
-No implementation stages have started. Add only concise stage commit references
-and blockers during execution; preserve detailed evidence in command output and
-the owning repository's Git history.
+- Stage 1 completed in `ansible-role-seriatim` commit `ac35c1f`.