test(domain): Registration aggregate invariants (refs #6)

Failing unit tests for the Registration aggregate root: a submission starts
in INGEDIEND carrying its bsn, an empty/whitespace/null bsn is rejected, the
started process-instance id is remembered, and attaching the zaak the ACL
opened records its URL idempotently (a conflicting URL is rejected) while the
status stays INGEDIEND.

The aggregate is a stub (no-op mutators, empty bsn) so the tests compile and
fail on their assertions; the green commit implements the invariants.

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

register-referentie.slnx

@@ -7,6 +7,10 @@
     <Project Path="services/acl/Acl.IntegrationTests/Acl.IntegrationTests.csproj" />
     <Project Path="services/acl/Acl.Tests/Acl.Tests.csproj" />
   </Folder>
+  <Folder Name="/services/domain/">
+    <Project Path="services/domain/Big.Domain/Big.Domain.csproj" />
+    <Project Path="services/domain/Big.Tests/Big.Tests.csproj" />
+  </Folder>
   <Folder Name="/services/bff/">
     <Project Path="services/bff/Bff.Api/Bff.Api.csproj" />
     <Project Path="services/bff/Bff.Tests/Bff.Tests.csproj" />