## What & why
Second sub-slice of **S-12 (#13)** — the **Workflow Client gains behandelaar user-task operations**, and the process model gains the beoordeling step.
- **BPMN:** `registratie.bpmn` now parks at a `Beoordelen` **userTask** (candidate group `behandelaar`) after `OpenZaakAanmaken`; `registrationId` rides along as a process variable so the werkbak can correlate each task to its aggregate.
- **Workflow Client** (`IUserTaskClient`, the only code that talks to Flowable §8.2):
- `GetOpenBeoordelingenAsync()` — the werkbak (open `Beoordelen` tasks + their `registrationId`)
- `ClaimAsync(taskId, behandelaar)`
- `CompleteBeoordelingAsync(taskId, besluit)` — carries the decision into the process as the `besluit` variable
- **Live integration:** `verify-domain` now drives the full user-task lifecycle against a real Flowable — after the worker opens the zaak, it polls for the task, claims it as `merel-behandelaar`, completes it (`goedkeuren`), and asserts the process finishes. This proves the exact REST contract (`service/runtime/tasks/query` + `…/{id}` claim/complete) the client depends on.
The walking skeleton is unaffected: the temporary `/approve` path still sets the zaak status directly; wiring the domain decision to *complete this task* (and driving the werkbak from the BFF) lands in **S-12c**.
## Definition of Done
- [x] Linked issue: #13 (umbrella; `refs`, does not close)
- [x] Tests first; red → green
- [x] Unit + acceptance green (`make unit`): domain 76, acceptance 9 (acl/event-subscriber/bff unaffected)
- [x] Mutation ≥ break(90): **domain 100%** (killed the new survivors *and* the pre-existing `FlowableWorkflowClient` baseline)
- [x] Live Flowable user-task lifecycle asserted in `verify-domain`
- [ ] CI green (pending)
Part of #13.
Reviewed-on: #83
