feat(domain): implement the Registration aggregate invariants (refs #6)

Submit requires a bsn and starts the aggregate in INGEDIEND; the started
process-instance id is recorded; AttachZaak stores the ACL's zaak URL,
tolerating a duplicate (at-least-once worker delivery) and rejecting a
conflicting URL, without advancing the status.

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

services/domain/Big.Tests/RegistrationTests.cs

@@ -21,7 +21,7 @@ public class RegistrationTests
     [InlineData("   ")]
     [InlineData(null)]
     public void Submitting_without_a_bsn_is_rejected(string? bsn)
-        => Assert.Throws<ArgumentException>(() => Registration.Submit(bsn!));
+        => Assert.ThrowsAny<ArgumentException>(() => Registration.Submit(bsn!));
 
     [Fact]
     public void Recording_the_started_process_keeps_its_instance_id()