feat(domain): Registration.Expire() lapses an open registration to Verlopen (refs #102)

Adds the terminal Verlopen status and Expire(), reusing the RequireOpenForDecision
guard so only an INGEDIEND/IN_BEHANDELING registration can lapse; idempotent once
Verlopen.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/domain/Big.Domain/RegistrationStatus.cs

@@ -21,4 +21,8 @@ public enum RegistrationStatus
 
     /// <summary>Withdrawn by the zorgprofessional before a decision (S-11). Terminal.</summary>
     Ingetrokken,
+
+    /// <summary>Lapsed: the required documents were not supplied within the 30-day window, so the
+    /// registratie process cancelled the case (S-10a). Terminal.</summary>
+    Verlopen,
 }