test(domain): store finds inscriptions due for a herregistratie reminder (refs #18)

New IRegistrationStore.FindDueForHerregistratieReminderAsync — the sweep's
candidate set. The production store is stubbed to an empty list so the new test
fails; the green commit filters on the aggregate's own reminder-due rule.

refs #18

services/domain/Big.Infrastructure/InMemoryRegistrationStore.cs

@@ -26,4 +26,8 @@ public sealed class InMemoryRegistrationStore : IRegistrationStore
     public Task<Registration?> FindOpenByBsnAsync(string bsn, CancellationToken ct = default)
         => Task.FromResult(_byId.Values.FirstOrDefault(r =>
             r.Bsn == bsn && r.Status is RegistrationStatus.Ingediend or RegistrationStatus.InBehandeling));
+
+    public Task<IReadOnlyList<Registration>> FindDueForHerregistratieReminderAsync(
+        DateTimeOffset asOf, CancellationToken ct = default)
+        => Task.FromResult<IReadOnlyList<Registration>>([]); // RED stub
 }