fix(calendar-sync): atomic writes, curl --fail, and zero-event vs garbage

Three robustness fixes from the config audit. (1) calendar-sync--write-file and --save-state now write a temp file in the same directory and rename it into place, so org-agenda or chime reading mid-write never sees a half-written calendar. (2) The two curl fetches gain --fail, so an HTTP 404/500 error page exits non-zero instead of flowing its HTML into conversion. (3) calendar-sync--parse-ics distinguishes a healthy zero-event calendar (a real iCalendar with BEGIN:VCALENDAR and no in-window events returns the header) from garbage (no VCALENDAR returns nil), so a near-empty calendar no longer reports "parse failed". New robustness tests; the empty-calendar boundary test updated to the corrected behavior. Verified against the live feed: all three calendars fetch and write cleanly.

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