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/Notification.cs

@@ -2,11 +2,16 @@ namespace EventSubscriber.Application;
 
 /// <summary>
 /// An inbound NRC (Open Notificaties) notification, as Open Notificaties POSTs it to an
-/// abonnement callback. Only the fields the projection needs are modelled; the full ZGW
-/// "Notificatie" resource also carries <c>aanmaakdatum</c> and <c>kenmerken</c> which the
-/// minimal projection ignores (bsn is deferred — see ADR-0008). For a <c>zaken</c>/<c>zaak</c>/<c>create</c>
-/// notification <c>hoofdObject</c> and <c>resourceUrl</c> are both the created zaak's URL.
+/// abonnement callback. Only the fields the projection needs are modelled.
 /// </summary>
+/// <remarks>
+/// Since S-19b-2 the subscriber listens on the <c>objecten</c> kanaal, not <c>zaken</c>: the
+/// register record in Objecten is what the projection is derived from (ADR-0030), so the
+/// projection is a cache of the register rather than a re-derivation of the case system. An
+/// Objecten notification carries <b>no record data</b> — only the object URL (as both
+/// <c>hoofdObject</c> and <c>resourceUrl</c>) and the objecttype as a kenmerk — so the record
+/// itself is read back through the ACL.
+/// </remarks>
 public sealed record Notification(
     string Kanaal,
     string Resource,
@@ -14,28 +19,12 @@ public sealed record Notification(
     Uri ResourceUrl,
     Uri? HoofdObject = null)
 {
-    /// <summary>A zaak being created — projected as INGEDIEND.</summary>
-    public bool IsZaakCreated =>
-        Kanaal == "zaken" && Resource == "zaak" && Actie == "create";
+    /// <summary>A register record written to Objecten — <c>create</c> on submit, <c>update</c> on
+    /// approval, since the ACL upserts the same object for a registration (§8.6).</summary>
+    public bool IsRegisterRecordWritten =>
+        Kanaal == "objecten" && Resource == "object" && Actie is "create" or "update";
 
-    /// <summary>A status being set on a zaak — the approval, projected as INGESCHREVEN (S-09b). In the
-    /// walking skeleton the only status ever set after creation is the approval, and the subscriber may
-    /// not read OpenZaak (§8.1), so any status-create is taken as the approval.</summary>
-    public bool IsZaakStatusSet =>
-        Kanaal == "zaken" && Resource == "status" && Actie == "create";
-
-    /// <summary>The zaak URL this notification concerns — <c>hoofdObject</c> (the zaak) for a status
-    /// notification, else the resource URL (which, for a zaak-create, is the zaak).</summary>
-    public Uri ZaakUrl => HoofdObject ?? ResourceUrl;
-
-    /// <summary>The zaak UUID used as the projection key — the trailing segment of <see cref="ZaakUrl"/>.</summary>
-    public string ZaakId => ZaakUrl.Segments[^1].Trim('/');
-
-    /// <summary>
-    /// A deterministic dedup key. Open Notificaties carries no notification id and may
-    /// redeliver, so the key is derived from the immutable notification content: two
-    /// deliveries of the same zaak-create collapse to one. (NRC may also deliver
-    /// out of order; the projector tolerates that — order does not change the outcome.)
-    /// </summary>
-    public string IdempotencyKey => $"{Kanaal}:{Resource}:{Actie}:{ResourceUrl}";
+    /// <summary>The object holding the register record. Objecten sets both fields to the object;
+    /// <c>hoofdObject</c> is the main resource by definition, so prefer it.</summary>
+    public Uri ObjectUrl => HoofdObject ?? ResourceUrl;
 }

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

@@ -3,28 +3,22 @@ namespace EventSubscriber.Application;
 /// <summary>
 /// Projects inbound NRC notifications into the read projection. Tolerates duplicate and
 /// out-of-order deliveries (CLAUDE.md §8.6): the notification log dedups, and the projection
-/// upsert is idempotent on the zaak id. Rebuilds the projection by replaying the log.
+/// upsert is idempotent on the register id. Rebuilds the projection by replaying the log.
 /// </summary>
 public sealed class NotificationProjector(INotificationLog log, IProjectionStore store, IAclClient acl)
 {
-    /// <summary>Handle one inbound notification. Reacts to a zaak being created (INGEDIEND) and a
-    /// status being set (INGESCHREVEN); ignores everything else. Enriches the row with the zaak's
-    /// reference via the ACL (§8.1) and records it so a rebuild needs no ZGW access (#78).</summary>
+    /// <summary>Handle one inbound notification. Reacts to a register record being written to
+    /// Objecten (S-19b-2, ADR-0030) and ignores everything else.</summary>
     public async Task HandleAsync(Notification notification, CancellationToken ct = default)
     {
-        if (!notification.IsZaakCreated && !notification.IsZaakStatusSet)
+        ArgumentNullException.ThrowIfNull(notification);
+
+        if (!notification.IsRegisterRecordWritten)
             return;
 
-        var reference = await acl.GetZaakReferenceAsync(notification.ZaakUrl, ct);
-        var recorded = new RecordedNotification(
-            notification.IdempotencyKey, notification.Actie, notification.ZaakId, notification.Resource, reference);
-
-        // Atomic record-or-skip: a duplicate (or concurrent) delivery is recognised and dropped
-        // before it touches the projection, so the projection stays a faithful derived artefact.
-        if (!await log.TryRecordAsync(recorded, ct))
-            return;
-
-        await store.UpsertAsync(ToEntry(recorded), ct);
+        // S-19b-2: reading the record back through the ACL and projecting it lands with the
+        // implementation; today nothing reaches the store.
+        await Task.CompletedTask;
     }
 
     /// <summary>Rebuild the projection from the durable notification log (PRD §8.4).</summary>
@@ -35,11 +29,9 @@ public sealed class NotificationProjector(INotificationLog log, IProjectionStore
             await store.UpsertAsync(ToEntry(recorded), ct);
     }
 
-    /// <summary>The projection row for an accepted notification: a status-set maps to INGESCHREVEN,
-    /// a zaak-create to INGEDIEND. bsn/naam are deferred (ADR-0008).</summary>
+    /// <summary>The projection row for an accepted notification. The log already holds exactly the
+    /// row's fields, so a rebuild needs no mapping rules and no upstream reads. bsn/naam stay
+    /// deferred — the register record is public-safe by construction (ADR-0027).</summary>
     private static RegisterEntry ToEntry(RecordedNotification recorded)
-        => new(
-            recorded.ZaakId,
-            recorded.Resource == "status" ? RegistrationStatus.Ingeschreven : RegistrationStatus.Ingediend,
-            Reference: recorded.Reference);
+        => new(recorded.RegisterId, recorded.Status, recorded.Reference);
 }

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