feat(domain): withdraw a registration → INGETROKKEN via POST /registrations/{id}/withdraw (refs #12)

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>

services/domain/Big.Domain/RegistrationStatus.cs

@@ -3,7 +3,8 @@ namespace Big.Domain;
 /// <summary>The lifecycle states a <see cref="Registration"/> moves through. Submission starts in
 /// <see cref="Ingediend"/>; a behandelaar takes it <see cref="InBehandeling"/> and decides it into one
 /// of the terminal states <see cref="Ingeschreven"/> (approved) or <see cref="Afgewezen"/> (rejected).
-/// Withdrawal and herregistratie states arrive in their own slices (S-11+).</summary>
+/// A zorgprofessional can withdraw a still-open registration into <see cref="Ingetrokken"/> (S-11).
+/// The herregistratie state arrives in its own slice.</summary>
 public enum RegistrationStatus
 {
     /// <summary>Submitted by the zorgprofessional; the registratie process has been started.</summary>
@@ -17,4 +18,7 @@ public enum RegistrationStatus
 
     /// <summary>Rejected by the behandelaar. Terminal.</summary>
     Afgewezen,
+
+    /// <summary>Withdrawn by the zorgprofessional before a decision (S-11). Terminal.</summary>
+    Ingetrokken,
 }