events: fix timezone-aware iCal export (local->UTC stamps) + 6 tests

Bug: tz events store wall-clock LOCAL times but export stamped them with a Z
(UTC) suffix, so a London 18:00 event falsely read as 18:00 UTC. ev-ical-conv
now converts a tz event's DTSTART/UNTIL/EXDATE/RDATE local->UTC before
formatting (London summer 18:00 -> 170000Z; Paris -> 160000Z); non-tz events
unchanged. Caveat: UTC RRULE drifts from wall-clock-stable tz recurrence across
a DST boundary (VTIMEZONE deferred). 366/366 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/events/scoreboard.json

@@ -1,12 +1,12 @@
 {
   "lang": "events",
-  "total_passed": 360,
+  "total_passed": 366,
   "total_failed": 0,
-  "total": 360,
+  "total": 366,
   "suites": [
     {"name":"calendar","passed":51,"failed":0,"total":51},
     {"name":"timezone","passed":17,"failed":0,"total":17},
-    {"name":"ical","passed":40,"failed":0,"total":40},
+    {"name":"ical","passed":46,"failed":0,"total":46},
     {"name":"availability","passed":22,"failed":0,"total":22},
     {"name":"api","passed":41,"failed":0,"total":41},
     {"name":"booking","passed":82,"failed":0,"total":82},
@@ -17,5 +17,5 @@
     {"name":"federation","passed":29,"failed":0,"total":29},
     {"name":"integration","passed":8,"failed":0,"total":8}
   ],
-  "generated": "2026-06-07T17:28:07+00:00"
+  "generated": "2026-06-07T18:33:58+00:00"
 }