feat(domain): record inscription date + herregistratie reminder-due rule (refs #18)

Approve stores the inscription moment; HerregistratieVoor derives the deadline
(inscription + 5-year validity); HerregistratieReminderDue(asOf) is true once the
16-week window before the deadline opens for a still-un-reminded inscription;
MarkHerregistratieReminderVerstuurd is idempotent and guards against reminding a
non-inscribed registration.

refs #18

services/domain/Big.Domain/Registration.cs

@@ -105,7 +105,7 @@ public sealed class Registration
 
         RequireOpenForDecision(nameof(Approve));
         Status = RegistrationStatus.Ingeschreven;
-        // RED stub: the inscription moment is not stored yet.
+        IngeschrevenOp = ingeschrevenOp;
     }
 
     // --- Herregistratie (S-17) — RED stubs, implemented in the green commit ---------------------
@@ -125,19 +125,35 @@ public sealed class Registration
 
     /// <summary>The date by which herregistratie must happen: inscription + validity. Null until
     /// inscribed.</summary>
-    public DateTimeOffset? HerregistratieVoor => null; // RED stub
+    public DateTimeOffset? HerregistratieVoor =>
+        IngeschrevenOp is DateTimeOffset ingeschrevenOp ? ingeschrevenOp + HerregistratieGeldigheid : null;
 
     /// <summary>Whether the herregistratie reminder has been sent for this inscription (S-17).</summary>
     public bool HerregistratieReminderVerstuurd { get; private set; }
 
     /// <summary>Whether, as of <paramref name="asOf"/>, this registration is due a herregistratie
     /// reminder: it is inscribed, the reminder window before its deadline has opened, and it has not
-    /// already been reminded.</summary>
-    public bool HerregistratieReminderDue(DateTimeOffset asOf) => false; // RED stub
+    /// already been reminded. Once inside the window it stays due until reminded (an overdue inscription
+    /// is still due). This is the single rule the store query and the sweep both build on.</summary>
+    public bool HerregistratieReminderDue(DateTimeOffset asOf) =>
+        Status == RegistrationStatus.Ingeschreven
+        && !HerregistratieReminderVerstuurd
+        && IngeschrevenOp is DateTimeOffset ingeschrevenOp
+        && asOf >= ingeschrevenOp + HerregistratieGeldigheid - Herinneringstermijn;
 
     /// <summary>Record that the herregistratie reminder has been sent. Idempotent — a re-sweep is a
-    /// no-op; only an inscribed registration can be reminded.</summary>
-    public void MarkHerregistratieReminderVerstuurd() { } // RED stub
+    /// no-op (§8.6); only an inscribed registration can be reminded.</summary>
+    public void MarkHerregistratieReminderVerstuurd()
+    {
+        if (HerregistratieReminderVerstuurd)
+            return;
+
+        if (Status != RegistrationStatus.Ingeschreven)
+            throw new InvalidOperationException(
+                $"Registration {Id} is {Status}; only an INGESCHREVEN registration can be sent a herregistratie reminder.");
+
+        HerregistratieReminderVerstuurd = true;
+    }
 
     /// <summary>
     /// Reject the registration — the behandelaar's decision not to enter it in the register. Advances a