Add the Ingetrokken terminal status, Registration.Withdraw() (open-only, idempotent), the
WithdrawRegistration handler, and the domain endpoint. Cancelling the running Flowable process
(so the case leaves the werkbak) and the owner-scoped BFF/self-service action are follow-up
sub-slices (S-11b/S-11c).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
