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.Tests/InMemoryRegistrationStoreTests.cs

@@ -79,7 +79,7 @@ public class InMemoryRegistrationStoreTests
         switch (transition)
         {
             case nameof(Registration.Withdraw): registration.Withdraw(); break;
-            case nameof(Registration.Approve): registration.Approve(); break;
+            case nameof(Registration.Approve): registration.Approve(DateTimeOffset.UtcNow); break;
             case nameof(Registration.Reject): registration.Reject(); break;
             case nameof(Registration.Expire): registration.Expire(); break;
         }