feat(render): iCalendar emitter, RFC 5545

Not a template job: folding, escaping, exclusive DTEND and stable UIDs
are rules a logic-less template cannot enforce, and each fails silently
in a subscriber's client rather than loudly at generation.

  DTEND is EXCLUSIVE for an all-day event (section 3.6.1). Wrong here
  shows every event a day short, everywhere.
  UIDs are YYYYMMDD-<rite>@colitur and stable across regenerations
  (section 3.8.4.7). Wrong here duplicates the whole year in every
  subscriber's phone, months later.
  Every line is CRLF-terminated and folded at 75 octets (section 3.1).
  No RRULE: a liturgical calendar is not a recurrence rule. Asserted, so
  nobody optimises it later.

DTSTAMP is a parameter, not a clock read. RFC 5545 requires it and the
obvious implementation reads the wall clock -- which violates the
kernel's determinism rule and would make two feeds from identical data
differ byte-for-byte, defeating reproducible builds and any reviewable
diff on a published tree.

Corrected one test literal against real engine output: DTSTAMP is a
per-VEVENT property (section 3.8.7.2), not calendar-level, so the
default-value line count is 365 (every event), not 1.

Mutation-tested: a non-exclusive DTEND reddens the suite.

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