test(domain): Workflow Client, ACL client, store and job processor (refs #6)

Failing infrastructure unit tests (stub HttpMessageHandler, fakes):
- FlowableWorkflowClient starts a process with the registrationId variable and
  returns the instance id; acquires OpenZaakAanmaken jobs (topic/workerId/lock)
  and parses their registrationId; completes a job with the zaakUrl variable —
  request URIs match flowable-rest's service/ and external-job-api/ paths.
- AclHttpClient POSTs the bsn to the ACL and returns the zaak URL.
- InMemoryRegistrationStore saves/reads/upserts by id.
- OpenZaakJobProcessor acquires, opens a zaak, completes the job; leaves a failing
  job uncompleted for redelivery; polls harmlessly when idle.

Adapters are stubs so the tests compile and fail on their assertions; the green
commit implements them against the REST contract verified on a live Flowable.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

register-referentie.slnx

@@ -10,6 +10,7 @@
   <Folder Name="/services/domain/">
     <Project Path="services/domain/Big.Domain/Big.Domain.csproj" />
     <Project Path="services/domain/Big.Application/Big.Application.csproj" />
+    <Project Path="services/domain/Big.Infrastructure/Big.Infrastructure.csproj" />
     <Project Path="services/domain/Big.Tests/Big.Tests.csproj" />
   </Folder>
   <Folder Name="/services/bff/">