… author)

- Add uppercase D tags to TimeBasedCalendarEvent for day-granularity relay filtering
- Change location from Option<String> to Vec<String> (spec says "repeated")
- Add optional author (p tag) to CalendarEventRsvp for event author pubkey
- Clean up RSVP deserialization to use Option instead of placeholder Coordinate

- M1: clarify `coordinates` doc comments as optional
- M2: validate YYYY-MM-DD format for DateBasedCalendarEvent start/end
- M4: return InvalidTimestamp instead of misleading StartMissing
- m3: document that event description lives in Nostr event content field
- m4: implement std::error::Error behind std feature flag
- Add tests for date validation, invalid timestamps, and error paths

- Add EventBuilder convenience methods for all 4 NIP-52 event kinds
- Replace Vec::new() with Vec::with_capacity() in tag serialization
- Remove std::error::Error impl to match NIP-53 consistency
- Fix RSVP status/FreeBusy parsing to return errors instead of silently skipping
- Add comments explaining uppercase:false match arms
- Add tests for multi-day D tags, RSVP with all fields, calendar with
  multiple coordinates, and unknown RSVP status error

…, add coverage gaps