feat(calendar-sync): add EXDATE support for excluded recurring event dates

When someone deletes a single instance of a recurring meeting in Google
Calendar, the calendar exports an EXDATE property marking that date as
excluded. Previously, calendar-sync expanded the RRULE without filtering
out these excluded dates, causing deleted instances to appear in org output.

New functions:
- calendar-sync--get-exdates: Extract all EXDATE values from event
- calendar-sync--get-exdate-line: Get full EXDATE line with parameters
- calendar-sync--parse-exdate: Parse EXDATE into datetime list
- calendar-sync--collect-exdates: Collect excluded dates with TZ conversion
- calendar-sync--exdate-matches-p: Check if occurrence matches an EXDATE
- calendar-sync--filter-exdates: Filter out excluded dates from occurrences

Modified calendar-sync--expand-recurring-event to collect and filter
EXDATEs after RRULE expansion.

Includes 47 new tests covering extraction, parsing, collection, filtering,
and integration with RECURRENCE-ID exceptions.

0 files changed, 0 insertions, 0 deletions