feat(domain): add the Ingeschreven state and Registration.Approve() (refs #75)

Approve() advances a submitted registration with an opened zaak to INGESCHREVEN;
re-approval and approval-before-zaak are rejected as invalid transitions.

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

services/domain/Big.Domain/Registration.cs

@@ -64,4 +64,23 @@ public sealed class Registration
 
         ZaakUrl = zaakUrl;
     }
+
+    /// <summary>
+    /// Approve the registration — the behandelaar's decision to enter it in the register. Advances
+    /// <see cref="RegistrationStatus.Ingediend"/> → <see cref="RegistrationStatus.Ingeschreven"/>.
+    /// Requires an opened zaak (the approval sets that zaak's status via the ACL), and only a
+    /// submitted registration can be approved: re-approving is rejected as an invalid transition.
+    /// </summary>
+    public void Approve()
+    {
+        if (ZaakUrl is null)
+            throw new InvalidOperationException(
+                $"Registration {Id} has no zaak yet; it cannot be approved before its zaak is opened.");
+
+        if (Status != RegistrationStatus.Ingediend)
+            throw new InvalidOperationException(
+                $"Registration {Id} is {Status}; only an INGEDIEND registration can be approved.");
+
+        Status = RegistrationStatus.Ingeschreven;
+    }
 }