test(domain): herregistratie inscription date + reminder-due rule (refs #18)

The Registration aggregate gains the herregistratie clock: Approve now records
the inscription moment, from which the herregistratie deadline and a
reminder-due rule are derived. Members are stubbed so the suite compiles and the
new assertions fail; the green commit implements them. Approve's signature gains
the inscription moment (callers now supply 'now' from an injected TimeProvider).

refs #18

services/domain/Big.Application/ApproveRegistration.cs

@@ -12,7 +12,7 @@ public sealed record ApproveRegistrationCommand(RegistrationId RegistrationId);
 /// zaak status is the projection's source of truth (it flows back over NRC); the aggregate transition
 /// keeps the domain's own view consistent.
 /// </summary>
-public sealed class ApproveRegistration(IRegistrationStore store, IAclClient acl)
+public sealed class ApproveRegistration(IRegistrationStore store, IAclClient acl, TimeProvider clock)
 {
     public async Task HandleAsync(ApproveRegistrationCommand command, CancellationToken ct = default)
     {
@@ -30,7 +30,7 @@ public sealed class ApproveRegistration(IRegistrationStore store, IAclClient acl
                 $"Registration {command.RegistrationId} has no zaak yet; it cannot be approved.");
 
         await acl.ApproveZaakAsync(registration.ZaakUrl, ct);
-        registration.Approve();
+        registration.Approve(clock.GetUtcNow());
         await store.SaveAsync(registration, ct);
     }
 }