test(acl): BDD acceptance scenario for opening a zaak (closes #5) (#49)

docs/architecture/adr-0004-bdd-framework.md

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+# ADR-0004: Reqnroll as the BDD acceptance framework
+
+- **Status:** Accepted
+- **Date:** 2026-06-04
+- **Deciders:** Respellion engineering
+- **Relates to:** S-04 (#5); supports CLAUDE.md §3 (BDD at the use-case level) and §11 (tests pyramid)
+
+## Context
+
+CLAUDE.md §11 mandates that each user-visible flow is driven by a Gherkin acceptance
+scenario living in `tests/acceptance/`, and §3 names "BDD at the use-case level" as a core
+engineering principle. The foundational slices (S-00…S-03) added no acceptance layer; S-04
+is the first slice with real domain behaviour to drive, so it is where the BDD framework is
+introduced. We need a .NET tool that:
+
+- parses Gherkin `.feature` files and binds steps to C#,
+- integrates with the existing xUnit test runner (the repo standardises on xUnit), so
+  acceptance tests run under the same `dotnet test` / `make ci` gate as everything else,
+- is actively maintained on modern .NET (we target net10.0).
+
+## Decision
+
+**Use [Reqnroll](https://reqnroll.net/) (`Reqnroll.xUnit`) for acceptance tests.**
+
+- Reqnroll is the actively-maintained, open-source successor to SpecFlow (which is no longer
+  maintained). It keeps the same Gherkin + `[Binding]` model, so the knowledge transfers.
+- `Reqnroll.xUnit` generates one xUnit test per scenario, so acceptance tests are discovered
+  and run by the same runner as the unit tests — no second test framework, no extra CI step.
+- Acceptance projects live under `tests/acceptance/` per the PRD §9 layout. Generated
+  `*.feature.cs` files are build artefacts and are git-ignored.
+
+## Consequences
+
+- **Positive:** one assertion/runner stack (xUnit) across unit and acceptance tests; scenarios
+  are written in business language (Dutch domain terms inline) and reviewed as the slice's
+  contract; maintained tooling on net10.0.
+- **Cost:** a new dependency (`Reqnroll.xUnit`) and its xUnit v2 transitive graph. Reqnroll
+  pulls `xunit.core` but not the assertion library, so the `xunit` metapackage is referenced
+  explicitly to get `Assert`.
+- **Replaceable by:** hand-written xUnit "scenario" tests with a Given/When/Then helper, at
+  the cost of losing Gherkin as the shared, readable contract — which is the whole point of §3.
+- **Follow-ups:** the real-OpenZaak integration test (Testcontainers) and the Stryker mutation
+  baseline for S-04 are tracked as their own issues split off #5.
+
+## Alternatives considered
+
+- **SpecFlow** — rejected: unmaintained and without an official net10.0 story; Reqnroll is its
+  drop-in successor.
+- **Plain xUnit Given/When/Then helpers** — rejected for user-visible flows: loses the
+  business-readable Gherkin contract that §3/§11 require. Still fine for unit-level tests.
+- **Xunit.Gherkin.Quick** — rejected: lighter but less featureful (no hooks/scoped contexts,
+  smaller community) than Reqnroll.