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>

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
 }

services/domain/Big.Tests/Fakes.cs

@@ -26,6 +26,11 @@ internal sealed class FakeRegistrationStore : IRegistrationStore
         => 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>>(
+            _byId.Values.Where(r => r.HerregistratieReminderDue(asOf)).ToList());
+
     public void Seed(Registration registration) => _byId[registration.Id] = registration;
 }
 

services/domain/Big.Tests/InMemoryRegistrationStoreTests.cs

@@ -96,4 +96,27 @@ public class InMemoryRegistrationStoreTests
 
         Assert.Null(await store.FindOpenByBsnAsync("123456782"));
     }
+
+    [Fact]
+    public async Task Finds_only_the_inscriptions_due_for_a_herregistratie_reminder()
+    {
+        var now = new DateTimeOffset(2026, 7, 23, 0, 0, 0, TimeSpan.Zero);
+        var store = new InMemoryRegistrationStore();
+
+        var due = Registration.Submit("123456782");
+        due.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/abc"));
+        due.Approve(now - Registration.HerregistratieGeldigheid + Registration.Herinneringstermijn);
+        await store.SaveAsync(due);
+
+        var freshlyInscribed = Registration.Submit("111111110");
+        freshlyInscribed.AttachZaak(new Uri("http://openzaak/zaken/api/v1/zaken/def"));
+        freshlyInscribed.Approve(now);
+        await store.SaveAsync(freshlyInscribed);
+
+        await store.SaveAsync(Registration.Submit("222222222")); // still INGEDIEND — never inscribed
+
+        var result = await store.FindDueForHerregistratieReminderAsync(now);
+
+        Assert.Equal([due.Id], result.Select(r => r.Id).ToArray());
+    }
 }