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/Registration.cs

@@ -111,7 +111,23 @@ public sealed class Registration
         Status = RegistrationStatus.Afgewezen;
     }
 
-    // A decision is only valid while the registration is still open (INGEDIEND or IN_BEHANDELING).
+    /// <summary>
+    /// Withdraw the registration — the zorgprofessional pulls their own submission back (S-11). Allowed
+    /// while it is still open (INGEDIEND or IN_BEHANDELING) and needs no zaak; a registration that has
+    /// already been decided (INGESCHREVEN/AFGEWEZEN) can no longer be withdrawn. Re-withdrawing one
+    /// already <see cref="RegistrationStatus.Ingetrokken"/> is a no-op.
+    /// </summary>
+    public void Withdraw()
+    {
+        if (Status == RegistrationStatus.Ingetrokken)
+            return;
+
+        RequireOpenForDecision(nameof(Withdraw));
+        Status = RegistrationStatus.Ingetrokken;
+    }
+
+    // A decision (or withdrawal) is only valid while the registration is still open (INGEDIEND or
+    // IN_BEHANDELING).
     private void RequireOpenForDecision(string decision)
     {
         if (Status is not (RegistrationStatus.Ingediend or RegistrationStatus.InBehandeling))