feat(domain): take a registration into behandeling; generalise decisions (refs #13)

Adds InBehandeling + Afgewezen statuses and TakeIntoBehandeling(). Generalises the
behandelaar decision: Approve() (requires a zaak) and new Reject() both act on an
INGEDIEND or IN_BEHANDELING registration, moving it to Ingeschreven/Afgewezen.

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

services/domain/Big.Domain/RegistrationStatus.cs

@@ -1,13 +1,20 @@
 namespace Big.Domain;
 
-/// <summary>The lifecycle states a <see cref="Registration"/> moves through. The walking
-/// skeleton knows <see cref="Ingediend"/> and the terminal <see cref="Ingeschreven"/>; withdrawal,
-/// beoordeling and herregistratie states arrive in their own slices (Iteration 2+).</summary>
+/// <summary>The lifecycle states a <see cref="Registration"/> moves through. Submission starts in
+/// <see cref="Ingediend"/>; a behandelaar takes it <see cref="InBehandeling"/> and decides it into one
+/// of the terminal states <see cref="Ingeschreven"/> (approved) or <see cref="Afgewezen"/> (rejected).
+/// Withdrawal and herregistratie states arrive in their own slices (S-11+).</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>
+    /// <summary>Picked up by a behandelaar for beoordeling (S-12).</summary>
+    InBehandeling,
+
+    /// <summary>Approved: entered in the register. Terminal.</summary>
     Ingeschreven,
+
+    /// <summary>Rejected by the behandelaar. Terminal.</summary>
+    Afgewezen,
 }