Model Registration status as a discriminated union

Each status variant now owns its own data: only Geregistreerd carries a
herregistratieDatum; Geschorst/Doorgehaald carry their own dates + reason.
A struck-off registration can no longer hold a future herregistratie date —
that impossible combination is gone from the type.

- status-badge keys color off the tag via a switch + assertNever
- registration-summary renders only the rows a variant's data supports
- registration.json nests the status; stories cover all three variants

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/mock/registration.json

@@ -2,8 +2,7 @@
   "bigNummer": "19012345601",
   "naam": "Dr. A. (Anna) de Vries",
   "beroep": "Arts",
-  "status": "Geregistreerd",
   "registratiedatum": "2012-09-01",
-  "herregistratieDatum": "2027-09-01",
-  "geboortedatum": "1985-03-14"
+  "geboortedatum": "1985-03-14",
+  "status": { "tag": "Geregistreerd", "herregistratieDatum": "2027-09-01" }
 }