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.Application/Ports.cs

@@ -107,6 +107,13 @@ public interface IRegistrationStore
     /// registration, or <c>null</c> if they have none in flight. Lets the self-service portal resume
     /// an existing registration after a refresh (S-26); terminal registrations are not resumed.</summary>
     Task<Registration?> FindOpenByBsnAsync(string bsn, CancellationToken ct = default);
+
+    /// <summary>The inscriptions whose herregistratie reminder is due as of <paramref name="asOf"/> and
+    /// not yet sent — the herregistratie reminder sweep's candidate set (S-17). The predicate is the
+    /// aggregate's own <see cref="Registration.HerregistratieReminderDue"/> rule, so the store never
+    /// duplicates the herregistratie policy.</summary>
+    Task<IReadOnlyList<Registration>> FindDueForHerregistratieReminderAsync(
+        DateTimeOffset asOf, CancellationToken ct = default);
 }
 
 /// <summary>