Add the iCalendar round-trip

Parsing goes through icalendar's low-level parser, which keeps properties
in order and keeps repeated ones. Writing is ours: that crate's writer
escapes a whole property value as text, so CATEGORIES:Work,Personal would
go out as one category named "Work,Personal" to every other client.

Anything the model does not interpret is carried rather than dropped --
X-MOZ-LASTACK, X-EVOLUTION-ALARM-UID, ACKNOWLEDGED, the X-MICROSOFT-CDO
set, unrecognised ATTENDEE parameters, and whole VTODO/VJOURNAL
components. A calendar has several clients writing to it and this one is
not the authority on which properties matter.

VTIMEZONE is modelled properly, and TZID is stored exactly as written:
Exchange names its zones "Pacific Standard Time", which no IANA lookup
resolves, and normalising at parse time would make the document
unrepresentable. Mapping to a real zone belongs at the point of use.

Tested against a golden corpus captured from the live Baikal (seven
producing clients over five years) and a published Outlook feed, scrubbed
of private content with the structure left byte-for-byte. Eight
hand-written fixtures cover what neither server had: DURATION, floating
times, RDATE, DST boundaries, leap day, and the full escape set.

The contract is that parse -> write -> parse is stable, plus a check that
no property name loses occurrences across the trip, since a parser that
dropped ATTENDEE entirely would round-trip perfectly and still be wrong.

Cargo.lock

@@ -2082,6 +2082,7 @@ dependencies = [
  "icalendar",
  "pretty_assertions",
  "rrule",
+ "runway-core",
  "serde",
  "serde_json",
  "thiserror 2.0.20",