test(acl): approval writes the RegisterRecord to Objecten (refs #149)

Ports and failing tests for the Objecten hop, ahead of the implementation:

- IRegisterRecordGateway + RegisterRecord — the Application-side port; the
  record mirrors the objecttype schema registered in S-18c (ADR-0027).
- AclService takes the port but does not yet call it, so the approval test
  fails on an empty upsert list.
- ObjectenGateway is a shell throwing NotImplementedException; its tests pin
  the contract: resolve the objecttype by name, search by data attribute,
  POST when absent / PATCH when present, static Token auth per API, the CRS
  headers the geo API requires, and a surfaced error body.

Also splits S-19 (#20) into #149/#150 in BACKLOG.md — the approval-side write
and the projection re-sourcing are independently deployable (CLAUDE.md §13).

tests/acceptance/Support/InMemoryZaakGateway.cs

@@ -53,3 +53,16 @@ public sealed class InMemoryZaakGateway : IZaakGateway
         => Task.FromResult<IReadOnlyList<ZaaktypeSummary>>(
             [new ZaaktypeSummary("BIG-REGISTRATIE", "BIG-registratie", ResolvedZaaktypeUrl)]);
 }
+
+/// <summary>An in-memory stand-in for the Objecten API (S-19a): records the register records the ACL
+/// writes on approval, so a scenario can assert on them without a running Objecten.</summary>
+public sealed class InMemoryRegisterRecordGateway : IRegisterRecordGateway
+{
+    public List<RegisterRecord> Upserted { get; } = [];
+
+    public Task UpsertAsync(RegisterRecord record, CancellationToken ct = default)
+    {
+        Upserted.Add(record);
+        return Task.CompletedTask;
+    }
+}