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.

crates/runway-core/tests/model.rs

@@ -443,7 +443,7 @@ fn series_with_one_override() -> CalendarObject {
         href: "/calendars/connor/personal/shared-uid.ics".to_owned(),
         etag: Some("\"abc123\"".to_owned()),
         calendar_path: "/calendars/connor/personal/".to_owned(),
-        events: vec![master, moved],
+        calendar: VCalendar::with_events(vec![master, moved]),
     }
 }
 
@@ -454,7 +454,7 @@ fn a_resource_separates_its_master_from_its_overrides() {
     assert!(object.master().is_some());
     assert_eq!(object.overrides().count(), 1);
     assert_eq!(
-        object.events.len(),
+        object.events().len(),
         2,
         "a master and its override are two VEVENTs in one resource, not a \
          duplicate to be deduplicated away",
@@ -472,7 +472,7 @@ fn every_event_in_a_resource_shares_one_uid() {
 #[test]
 fn a_resource_with_mismatched_uids_is_detected() {
     let mut object = series_with_one_override();
-    object.events[1].uid = "different".to_owned();
+    object.calendar.events[1].uid = "different".to_owned();
 
     assert!(
         !object.has_consistent_uid(),
@@ -484,7 +484,7 @@ fn a_resource_with_mismatched_uids_is_detected() {
 #[test]
 fn a_resource_holding_only_overrides_has_no_master() {
     let mut object = series_with_one_override();
-    object.events.retain(VEvent::is_override);
+    object.calendar.events.retain(VEvent::is_override);
 
     assert!(object.master().is_none());
     assert_eq!(object.overrides().count(), 1);