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

The Registration aggregate gains the herregistratie clock: Approve now records
the inscription moment, from which the herregistratie deadline and a
reminder-due rule are derived. Members are stubbed so the suite compiles and the
new assertions fail; the green commit implements them. Approve's signature gains
the inscription moment (callers now supply 'now' from an injected TimeProvider).

refs #18

services/domain/Big.Domain/Registration.cs

@@ -92,11 +92,12 @@ public sealed class Registration
 
     /// <summary>
     /// Approve the registration — the behandelaar's decision to enter it in the register. Advances a
-    /// submitted or in-behandeling registration to <see cref="RegistrationStatus.Ingeschreven"/>.
-    /// Requires an opened zaak (the approval sets that zaak's status via the ACL); a registration that
-    /// has already been decided cannot be approved again.
+    /// submitted or in-behandeling registration to <see cref="RegistrationStatus.Ingeschreven"/> and
+    /// records <paramref name="ingeschrevenOp"/> as the moment of inscription, which starts the
+    /// herregistratie clock (S-17). Requires an opened zaak (the approval sets that zaak's status via
+    /// the ACL); a registration that has already been decided cannot be approved again.
     /// </summary>
-    public void Approve()
+    public void Approve(DateTimeOffset ingeschrevenOp)
     {
         if (ZaakUrl is null)
             throw new InvalidOperationException(
@@ -104,8 +105,40 @@ public sealed class Registration
 
         RequireOpenForDecision(nameof(Approve));
         Status = RegistrationStatus.Ingeschreven;
+        // RED stub: the inscription moment is not stored yet.
     }
 
+    // --- Herregistratie (S-17) — RED stubs, implemented in the green commit ---------------------
+
+    /// <summary>How long a BIG inscription stays valid before herregistratie is required.</summary>
+    // ponytail: fixed 5-year term — a calibration knob, not a config surface. If a demo needs it
+    // per-catalogus, promote it to policy passed in from the beheer config (S-15).
+    public static readonly TimeSpan HerregistratieGeldigheid = TimeSpan.FromDays(365 * 5);
+
+    /// <summary>How long before the deadline the herregistratie reminder is sent (BIG: ~16 weeks).</summary>
+    // ponytail: fixed 16-week lead time — calibration knob; same promotion path as HerregistratieGeldigheid.
+    public static readonly TimeSpan Herinneringstermijn = TimeSpan.FromDays(16 * 7);
+
+    /// <summary>When the registration was entered in the register, once approved; the start of its
+    /// herregistratie clock. Null until it is <see cref="RegistrationStatus.Ingeschreven"/>.</summary>
+    public DateTimeOffset? IngeschrevenOp { get; private set; }
+
+    /// <summary>The date by which herregistratie must happen: inscription + validity. Null until
+    /// inscribed.</summary>
+    public DateTimeOffset? HerregistratieVoor => null; // RED stub
+
+    /// <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
+
+    /// <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
+
     /// <summary>
     /// Reject the registration — the behandelaar's decision not to enter it in the register. Advances a
     /// submitted or in-behandeling registration to <see cref="RegistrationStatus.Afgewezen"/>. Unlike