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

Introduce a Reqnroll.xUnit acceptance project under tests/acceptance/ and
the first Gherkin scenario driving S-04: the ACL default-fills the
ZGW-mandatory zaak fields (ADR-0003) from a domain registration payload.

Red: the scenario is discovered but has no step bindings yet, so it fails
with undefined steps. The green commit wires the bindings to AclService.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -5,6 +5,9 @@ obj/
 [Rr]elease/
 *.user
 
+# Reqnroll-generated test code (regenerated from *.feature on build)
+*.feature.cs
+
 # Test results / coverage
 [Tt]est[Rr]esults/
 *.trx