feat(#13): S-12a — beoordeling decision model (domain) (#82)

## What & why

First sub-slice of **S-12 (#13)** — the **beoordeling decision model** in the Domain Service. Foundation for the behandel-portal: it gives the domain a proper decision lifecycle before any UI/Flowable/BFF work.

- **Statuses:** add `InBehandeling` and `Afgewezen` to `RegistrationStatus`.
- **Aggregate:** `TakeIntoBehandeling()` (`Ingediend → InBehandeling`, idempotent, guards terminal states); generalise the behandelaar decision — `Approve()` (requires a zaak) and new `Reject()` both act on an `Ingediend`/`InBehandeling` registration → `Ingeschreven`/`Afgewezen`.
- **Use-case:** `BeoordeelRegistratie` (`goedkeuren` sets the zaak's final status via the ACL §8.1 → `Ingeschreven`; `afwijzen` → `Afgewezen`, domain-only for now). Idempotent.
- **Endpoint:** `POST /registrations/{id}/decide` (`{ "besluit": "goedkeuren" | "afwijzen" }`), superseding the temporary `/approve` (retired when the portal lands, S-12d).
- **BDD:** `EenRegistratieBeoordelen.feature` — goedkeuren + afwijzen scenarios (feature-scoped bindings).

**Scoped out** to later S-12 sub-slices: Flowable user-task claim/complete + BPMN `userTask` (S-12b), BFF `/behandel/*` + medewerker authz (S-12c), the Angular behandel-portal + e2e (S-12d), and propagating a *rejection* to the zaak/projection via the ACL.

## Definition of Done

- [x] Linked issue: #13 (umbrella; this PR `refs`, does not close)
- [x] Tests first; red → green per behaviour
- [x] Unit + acceptance green (`make unit`): domain 65, acceptance 9
- [x] Mutation ≥ break(90): domain 98.77%, no survivors in new code (the one unkilled mutant is the pre-existing `FlowableWorkflowClient` baseline)
- [ ] CI green (pending)

Part of #13.

Reviewed-on: #82

tests/acceptance/Features/EenRegistratieBeoordelen.feature

@@ -0,0 +1,24 @@
+# language: en
+# Drives S-12 (#13). A behandelaar picks up a submitted registration for beoordeling and decides it:
+# goedkeuren enters it in the register (INGESCHREVEN, the zaak's final status set via the ACL, §8.1),
+# afwijzen turns it down (AFGEWEZEN). This scenario exercises the use cases against in-memory
+# stand-ins for the ACL and the store; real Flowable user-task claim/complete arrives in a later
+# sub-slice and is verified live.
+Feature: Een registratie beoordelen
+  Als behandelaar wil ik een ingediende registratie beoordelen
+  zodat deze wordt ingeschreven of afgewezen.
+
+  Scenario: Goedkeuren schrijft de registratie in via de ACL
+    Given a submitted registration with an opened zaak
+    When the behandelaar takes it into behandeling
+    Then the registration has status "INBEHANDELING"
+    When the behandelaar decides "goedkeuren"
+    Then the registration has status "INGESCHREVEN"
+    And the zaak's final status is set via the ACL
+
+  Scenario: Afwijzen wijst de registratie af zonder de ACL
+    Given a submitted registration with an opened zaak
+    When the behandelaar takes it into behandeling
+    And the behandelaar decides "afwijzen"
+    Then the registration has status "AFGEWEZEN"
+    And the ACL is not asked to set the zaak status

tests/acceptance/Steps/EenRegistratieBeoordelenSteps.cs

@@ -0,0 +1,58 @@
+using Acceptance.Support;
+using Big.Application;
+using Big.Domain;
+using Reqnroll;
+using Xunit;
+
+namespace Acceptance.Steps;
+
+/// <summary>Bindings for <c>EenRegistratieBeoordelen.feature</c> (S-12). Drives the TakeIntoBehandeling
+/// transition and the BeoordeelRegistratie use case against in-memory ports; one instance per scenario.
+/// Scoped to this feature so its "the registration has status" step does not clash with the identically
+/// phrased (but differently-asserting) step in <see cref="RegistratieIndienenSteps"/>.</summary>
+[Binding]
+[Scope(Feature = "Een registratie beoordelen")]
+public sealed class EenRegistratieBeoordelenSteps
+{
+    private readonly InMemoryAclClient _acl = new();
+    private readonly InMemoryRegistrationStore _store = new();
+    private RegistrationId _id;
+
+    [Given("a submitted registration with an opened zaak")]
+    public async Task GivenASubmittedRegistrationWithAnOpenedZaak()
+    {
+        var registration = Registration.Submit("123456782");
+        registration.AttachZaak(InMemoryAclClient.OpenedZaakUrl);
+        await _store.SaveAsync(registration);
+        _id = registration.Id;
+    }
+
+    [When("the behandelaar takes it into behandeling")]
+    public async Task WhenTheBehandelaarTakesItIntoBehandeling()
+    {
+        var registration = await _store.GetAsync(_id);
+        registration!.TakeIntoBehandeling();
+        await _store.SaveAsync(registration);
+    }
+
+    [When("the behandelaar decides \"(.*)\"")]
+    public async Task WhenTheBehandelaarDecides(string besluit)
+        => await new BeoordeelRegistratie(_store, _acl).HandleAsync(
+            new BeoordeelRegistratieCommand(_id, Enum.Parse<BeoordelingsBesluit>(besluit, ignoreCase: true)));
+
+    [Then("the registration has status \"(.*)\"")]
+    public async Task ThenTheRegistrationHasStatus(string expected)
+    {
+        var registration = await _store.GetAsync(_id);
+        Assert.NotNull(registration);
+        Assert.Equal(expected, registration.Status.ToString().ToUpperInvariant());
+    }
+
+    [Then("the zaak's final status is set via the ACL")]
+    public void ThenTheZaakFinalStatusIsSetViaTheAcl()
+        => Assert.Equal(InMemoryAclClient.OpenedZaakUrl, _acl.ApprovedZaakUrl);
+
+    [Then("the ACL is not asked to set the zaak status")]
+    public void ThenTheAclIsNotAskedToSetTheZaakStatus()
+        => Assert.Null(_acl.ApprovedZaakUrl);
+}