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/Steps/EenZaakOpenenSteps.cs

@@ -46,7 +46,7 @@ public sealed class EenZaakOpenenSteps
     {
         var fill = new InMemoryDefaultFillStore(new DefaultFillSettings(
             _defaults!.Bronorganisatie, _defaults.VerantwoordelijkeOrganisatie, _defaults.Vertrouwelijkheidaanduiding));
-        var service = new AclService(_gateway, fill, new CachedZaaktypeCatalog(_gateway, _defaults!), new FixedClock(_today));
+        var service = new AclService(_gateway, new InMemoryRegisterRecordGateway(), fill, new CachedZaaktypeCatalog(_gateway, _defaults!), new FixedClock(_today));
         _returnedUrl = await service.OpenZaakAsync(_registration!);
     }