test(event-subscriber): project zaak-created notifications into the read projection (refs #7)

Failing unit + acceptance tests for the Event Subscriber's NotificationProjector:
a zaken/zaak/create notification yields one INGEDIEND projection row, duplicate
deliveries collapse to one row, non-zaak/non-create notifications are ignored, and
a rebuild repopulates the projection from the durable notification log (PRD §8.4).

The projector is a no-op stub so the tests compile and fail on the assertions; the
implementation follows in the green commit. The notification log doubles as the
idempotency guard and rebuild source so a rebuild needs no OpenZaak access (§8.1).

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

services/event-subscriber/EventSubscriber.Application/RegisterEntry.cs

@@ -0,0 +1,20 @@
+namespace EventSubscriber.Application;
+
+/// <summary>
+/// A row of the rebuildable read projection (PRD §8.4). For the minimal slice it carries
+/// the zaak id and a status; <c>Bsn</c> and <c>NaamPlaceholder</c> are part of the schema but
+/// are deferred — the NRC notification does not carry them and the subscriber may not read
+/// OpenZaak directly (CLAUDE.md §8.1). Populating them is a follow-up (ADR-0008).
+/// </summary>
+public sealed record RegisterEntry(
+    string Id,
+    string Status,
+    string? Bsn = null,
+    string? NaamPlaceholder = null);
+
+/// <summary>The projection statuses the walking skeleton knows about.</summary>
+public static class RegistrationStatus
+{
+    /// <summary>A zaak has been created; the registration is submitted.</summary>
+    public const string Ingediend = "INGEDIEND";
+}