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/conformance.conf

@@ -18,6 +18,7 @@ PRELOADS=(
   lib/datalog/api.sx
   lib/datalog/magic.sx
   lib/events/calendar.sx
+  lib/events/timezone.sx
   lib/events/availability.sx
   lib/persist/event.sx
   lib/persist/backend.sx
@@ -47,6 +48,7 @@ PRELOADS=(
 
 SUITES=(
   "calendar:lib/events/tests/calendar.sx:(ev-calendar-tests-run!)"
+  "timezone:lib/events/tests/timezone.sx:(ev-timezone-tests-run!)"
   "availability:lib/events/tests/availability.sx:(ev-availability-tests-run!)"
   "api:lib/events/tests/api.sx:(ev-api-tests-run!)"
   "booking:lib/events/tests/booking.sx:(ev-booking-tests-run!)"