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>

services/domain/Big.Domain/RegistrationId.cs

@@ -0,0 +1,15 @@
+namespace Big.Domain;
+
+/// <summary>The identity of a <see cref="Registration"/> aggregate — opaque, server-assigned,
+/// and distinct from the OpenZaak zaak id (which the projection keys on). A value object so the
+/// id can travel as a Flowable process variable and back without ever being a bare string.</summary>
+public readonly record struct RegistrationId(Guid Value)
+{
+    /// <summary>Mint a fresh identity for a newly submitted registration.</summary>
+    public static RegistrationId New() => new(Guid.NewGuid());
+
+    /// <summary>Rehydrate an id carried as a string (e.g. a Flowable process variable).</summary>
+    public static RegistrationId Parse(string value) => new(Guid.Parse(value));
+
+    public override string ToString() => Value.ToString();
+}