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/RegistrationStatus.cs

@@ -1,10 +1,13 @@
 namespace Big.Domain;
 
 /// <summary>The lifecycle states a <see cref="Registration"/> moves through. The walking
-/// skeleton knows only <see cref="Ingediend"/>; withdrawal, beoordeling and herregistratie
-/// states arrive in their own slices (Iteration 2+).</summary>
+/// skeleton knows <see cref="Ingediend"/> and the terminal <see cref="Ingeschreven"/>; withdrawal,
+/// beoordeling and herregistratie states arrive in their own slices (Iteration 2+).</summary>
 public enum RegistrationStatus
 {
     /// <summary>Submitted by the zorgprofessional; the registratie process has been started.</summary>
     Ingediend,
+
+    /// <summary>Approved: entered in the register. Terminal in the walking skeleton (S-09b).</summary>
+    Ingeschreven,
 }