events: timezone + DST support + 17 tests

timezone.sx: wall-clock LOCAL <-> absolute UTC. :fixed + :dst zones (std/dst
offsets + UTC transition rules, EU-style, no IANA DB) computed via calendar
helpers. ev-event-tz authors in local time; ev-expand expands tz events in
LOCAL time then converts each occurrence to UTC, so a 09:00 weekly meeting
stays 09:00 across a DST change (UTC instant shifts). Predefined utc/london/
paris. Plain events unaffected. 295/295 green.

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

lib/events/scoreboard.json

@@ -1,10 +1,11 @@
 {
   "lang": "events",
-  "total_passed": 278,
+  "total_passed": 295,
   "total_failed": 0,
-  "total": 278,
+  "total": 295,
   "suites": [
     {"name":"calendar","passed":51,"failed":0,"total":51},
+    {"name":"timezone","passed":17,"failed":0,"total":17},
     {"name":"availability","passed":22,"failed":0,"total":22},
     {"name":"api","passed":24,"failed":0,"total":24},
     {"name":"booking","passed":82,"failed":0,"total":82},
@@ -14,5 +15,5 @@
     {"name":"reminders","passed":21,"failed":0,"total":21},
     {"name":"federation","passed":29,"failed":0,"total":29}
   ],
-  "generated": "2026-06-07T08:12:04+00:00"
+  "generated": "2026-06-07T09:30:28+00:00"
 }