Splits backend CallerIdentity into the two ADR-0002 §3 actor kinds (ZorgverlenerCaller/MedewerkerCaller), a stub X-Medewerker/X-Rollen header path mirroring WP-53's citizen stub, and Authz.CanBeoordelen as the first medewerker capability — backend-only, no consumer until WP-64. Also fixes the backlog README's stale WP-61 status (done, but table said todo). Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>