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"
   }

apps/api/src/broker/expand.ts

@@ -27,7 +27,8 @@ import ICAL from 'ical.js'
  * id: stable identity key = `${uid}::${startIso}` — Schedule-X uses this for dedup.
  * start/end:
  *   - All-day:  'YYYY-MM-DD' (DATE string, no time component) — must use Temporal.PlainDate on client
- *   - Timed:    offset-aware ISO-8601 e.g. '2026-06-01T10:00:00-04:00' — use Temporal.ZonedDateTime on client
+ *   - Timed:    IANA-annotated ISO-8601 e.g. '2026-06-01T10:00:00-04:00[America/New_York]' —
+ *               Temporal.ZonedDateTime.from() requires the IANA bracket; offset-only strings throw.
  *
  * Color routing (D-06, CAL-02):
  *   The client routes calendarId for Schedule-X as: isShared ? 'shared' : String(ownerUserId)
@@ -49,9 +50,9 @@ export interface CalendarOccurrence {
   /** True when this occurrence belongs to the shared-family calendar (calendars.isShared=true) */
   isShared: boolean
   title: string
-  /** 'YYYY-MM-DD' for all-day events; offset-aware ISO string for timed events */
+  /** 'YYYY-MM-DD' for all-day events; IANA-annotated ISO string for timed events e.g. '2026-06-01T10:00:00-04:00[America/New_York]' */
   start: string
-  /** 'YYYY-MM-DD' for all-day events; offset-aware ISO string for timed events */
+  /** 'YYYY-MM-DD' for all-day events; IANA-annotated ISO string for timed events e.g. '2026-06-01T11:00:00-04:00[America/New_York]' */
   end: string
   allDay: boolean
   location: string | null
@@ -84,10 +85,12 @@ function formatUtcOffset(offsetSeconds: number): string {
 }
 
 /**
- * Serialize an ICAL.Time to an offset-aware ISO 8601 string for timed events,
+ * Serialize an ICAL.Time to an IANA-annotated ISO 8601 string for timed events,
  * or a plain 'YYYY-MM-DD' string for all-day events.
  *
- * For timed events: '2026-03-01T10:00:00-05:00'
+ * For timed events: '2026-03-01T10:00:00-05:00[America/New_York]'
+ *   The IANA bracket is REQUIRED — Temporal.ZonedDateTime.from() throws on offset-only
+ *   strings such as '2026-03-01T10:00:00-05:00'. See verified diagnosis in PLAN.md.
  * For all-day events: '2026-06-15'
  */
 function serializeTime(t: ICAL.Time, allDay: boolean): string {
@@ -99,17 +102,27 @@ function serializeTime(t: ICAL.Time, allDay: boolean): string {
     return `${y}-${m}-${d}`
   }
 
-  // Timed: build an offset-aware ISO string so the client can construct Temporal.ZonedDateTime
-  // ICAL.Time.toString() gives 'YYYY-MM-DDTHH:mm:ss' (no offset) — we append the offset.
-  const base = t.toString() // e.g. '2026-03-01T10:00:00'
+  // Timed: build an IANA-annotated ISO string so the client can construct Temporal.ZonedDateTime.
+  // ICAL.Time.toString() gives 'YYYY-MM-DDTHH:mm:ssZ' for UTC or 'YYYY-MM-DDTHH:mm:ss' for local.
+  // We always emit '...±HH:MM[IANA/Zone]' — the bracket is mandatory for ZonedDateTime.from().
   if (t.zone === ICAL.Timezone.utcTimezone) {
-    // UTC zone: toString() appends 'Z' already — but toString() doesn't do this.
-    // We handle it explicitly.
-    return base + 'Z'
+    // UTC zone: toString() produces '...Z'; strip the Z and emit +00:00[UTC].
+    const base = t.toString().replace(/Z$/, '') // e.g. '2026-03-01T10:00:00'
+    return `${base}+00:00[UTC]`
   }
+
+  const tzid = t.zone?.tzid
+  // Floating zone (tzid === 'floating') has no real timezone — fall back to UTC.
+  // This is a safe degradation: the event had no VTIMEZONE and no offset is knowable.
+  if (!tzid || tzid === 'floating') {
+    const base = t.toString() // no trailing Z for floating
+    return `${base}+00:00[UTC]`
+  }
+
+  // Named IANA zone: combine base datetime + offset + IANA bracket.
+  const base = t.toString() // e.g. '2026-03-01T10:00:00'
   const offsetSec = t.utcOffset()
-  // If zone is floating (no registered timezone), utcOffset() returns 0 — treat as UTC
-  return base + formatUtcOffset(offsetSec)
+  return `${base}${formatUtcOffset(offsetSec)}[${tzid}]`
 }
 
 /**