test(event-subscriber): the projection is sourced from register records (refs #153)

Ports, schema and failing tests for the subscriber half of S-19b-2, ahead of the
implementation.

The subscriber now listens on the `objecten` kanaal instead of `zaken`. An Objecten
notification carries no record data — only the object URL — so the record is read back
through the ACL (§8.1), and the zaak-shaped surface goes away: IsZaakCreated /
IsZaakStatusSet / ZaakUrl / ZaakId and ToEntry's `Resource == "status"` mapping are replaced
by IsRegisterRecordWritten + ObjectUrl.

The notification log now holds the projected row itself (register id, status, reference),
so a rebuild is a replay with no mapping rules and no upstream reads. The migration drops
the old columns rather than renaming them — EF scaffolded renames that would have carried
ZGW values into columns meaning something else — and empties both tables, since a
pre-slice row is neither reprojectable nor re-derivable from the new source.

Red: HandleAsync recognises a register write but does not yet read or project it, so the
seven projection assertions fail on an empty store.

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

@@ -4,7 +4,7 @@ namespace EventSubscriber.Application;
 /// The durable log of notifications the subscriber has accepted. It is both the idempotency
 /// guard (a replayed notification is recognised and dropped) and the rebuild source: the
 /// projection is a derived artefact (PRD §8.4) regenerated by replaying this log, so a rebuild
-/// needs no access to OpenZaak (CLAUDE.md §8.1). Implemented in Infrastructure over Postgres.
+/// needs no access to Objecten or ZGW (CLAUDE.md §8.1). Implemented in Infrastructure over Postgres.
 /// </summary>
 public interface INotificationLog
 {
@@ -19,22 +19,29 @@ public interface INotificationLog
     Task<IReadOnlyList<RecordedNotification>> AllAsync(CancellationToken ct = default);
 }
 
-/// <summary>A notification that has been accepted, retaining what a rebuild needs to recompute its
-/// projection row — the ZGW <c>resource</c> (zaak-create → INGEDIEND vs status-set → INGESCHREVEN) and
-/// the zaak <c>reference</c> (identificatie), so a rebuild reproduces the row without re-reading ZGW (#78).</summary>
-public sealed record RecordedNotification(string Key, string Actie, string ZaakId, string Resource, string? Reference);
+/// <summary>
+/// An accepted notification, retaining exactly the projection row it produced — so a rebuild
+/// reproduces the row by replaying the log, without re-reading Objecten (S-19b-2, ADR-0030).
+/// </summary>
+public sealed record RecordedNotification(string Key, string RegisterId, string Status, string? Reference);
 
 /// <summary>
-/// Port to the Anti-Corruption Layer. The subscriber enriches the projection with the zaak's
-/// public-safe reference (its identificatie) by asking the ACL — the only code that may read ZGW
-/// (§8.1) — rather than reading OpenZaak itself (adr-proposal #78).
+/// Port to the Anti-Corruption Layer. An Objecten notification carries only the object URL, so the
+/// subscriber reads the register record back through the ACL — the only code that may talk to
+/// Objecten (§8.1, ADR-0028) — rather than reading Objecten itself.
 /// </summary>
 public interface IAclClient
 {
-    /// <summary>The zaak's reference (identificatie) for the read projection.</summary>
-    Task<string> GetZaakReferenceAsync(Uri zaakUrl, CancellationToken ct = default);
+    /// <summary>The register record the object at <paramref name="objectUrl"/> holds, or
+    /// <c>null</c> if it holds none — the object may be gone by the time a redelivered
+    /// notification is handled, which is not an error (§8.6).</summary>
+    Task<RegisterRecord?> GetRegisterRecordAsync(Uri objectUrl, CancellationToken ct = default);
 }
 
+/// <summary>The public-safe register record as the ACL returns it — the RegisterRecord objecttype's
+/// schema (ADR-0027). No bsn, no name: the register is world-readable.</summary>
+public sealed record RegisterRecord(string Id, string Status, string? Reference);
+
 /// <summary>The read projection store. Owned by the projection bounded context (ADR-0008); the
 /// subscriber writes to it and the projection-api reads it.</summary>
 public interface IProjectionStore