[tobixen]

Commit message:

feat: cancel() marks an object as cancelled

cancel() sets STATUS:CANCELLED and saves, on the overrides of a series
too.  On a single occurrence from search(expand=True) it becomes a
cancelled override in the series, for an occurrence that was planned
but will not take place.  Zimbra turns
such an override into an EXDATE; a new compatibility feature,
save-load.event.recurrences.exception.cancelled, records that.

Split out of the work on taking an occurrence out of a series with
delete(), which is postponed.  Part of
https://github.com/python-caldav/caldav/issues/598

Prompt: Fix 782, 799, 598, 795, 805, 806 [python-caldav issues; this commit is https://github.com/python-caldav/caldav/issues/598].  The small ones can share one PR, the bigger ones should probably have separated PRs.  (...)
Followup-Prompt: Check the updates on the issue - the EXDATE and .delete()-logic here will be procrastinated.  However, this branch also implements `.cancel()` and a _delete_headers refacotring (sic) which is "needed".  (that's what the other session said - it's 391k tokens big, so it's probably cheaper to work on this from a clean context?).  Please create a new branch for `.cancel()`, and consider if the `sync/async twin _delete_headers`-refactoring is also needed.  Push as a PR referencing but not closing the issue.  Rebase and push the original branch (no review needed)

Assisted-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Reviewed-by: Tobias Brox <tobias@redpill-linpro.com>

The EXDATE logic for delete() on an occurrence stays parked on https://github.com/python-caldav/caldav/tree/feat/delete-occurrence-598, now rebased on top of this branch.