fix(02): emit IANA-annotated timed strings from serializeTime

Temporal.ZonedDateTime.from() rejects offset-only ISO strings such as
'2026-06-18T08:00:00-04:00'; it requires an IANA bracket, e.g.
'2026-06-18T08:00:00-04:00[America/New_York]'. serializeTime() was
emitting offset-only for named zones and bare 'Z' for UTC — both
unparseable by the frontend, blanking the calendar view.

Changes:
- Named IANA zone: emit '...±HH:MM[tzid]' using t.zone.tzid
- UTC zone: strip trailing 'Z' from toString(), emit '+00:00[UTC]'
- Floating zone (no registered VTIMEZONE): fall back to '+00:00[UTC]'
- Update CalendarOccurrence docstrings to reflect the IANA-annotated contract
- Add temporal-polyfill@0.3.2 as dev dep in api for cross-contract test

apps/api/package.json

@@ -29,6 +29,7 @@
   "devDependencies": {
     "@types/node": "^22.0.0",
     "drizzle-kit": "0.31.10",
+    "temporal-polyfill": "0.3.2",
     "typescript": "^5.5.0",
     "vitest": "^4.1.8"
   }