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).

BACKLOG.md

@@ -287,12 +287,17 @@ Split into independently deployable sub-slices (CLAUDE.md §13):
 - **S-18b** (#140, ✅) · Objecten API up in compose, wired to Objecttypen. Depends on S-18a.
 - **S-18c** (#141, ✅) · RegisterRecord objecttype defined + registered (public-safe JSON schema). Depends on S-18a/b.
 
-### S-19 · ACL extension: write register-record to Objecten on approval
+### S-19 · ACL extension: write register-record to Objecten on approval *(split — #20 closed)*
 
 **Outcome:** Approval path writes the canonical register record to Objecten, not OpenZaak eigenschappen. Projection now sourced from Objecten events.
 
 **ADR required:** "Why Objecten holds the register, OpenZaak holds the process."
 
+Split into independently deployable sub-slices (CLAUDE.md §13):
+
+- **S-19a** (#149) · ACL writes the `RegisterRecord` to Objecten on approval, idempotently, alongside the ZGW eindstatus. Carries the ADR.
+- **S-19b** (#150) · Read projection sourced from Objecten instead of NRC zaak events. Depends on S-19a.
+
 ---
 
 ## Iteration 5 — Data governance module *(milestone: `Iteration 5 — Data Governance`)*