Recurrence override full fidelity

[SashankBhamidi]

Per-instance changes in recurring events are silently dropped. The converter only preserves title, start, duration, and description in recurrence overrides — everything else (participants, locations, alerts, status, keywords) is lost on round-trip.

This means a recurring meeting where one instance has different attendees, a different location, or a modified alert will silently lose those changes. It's a data integrity bug in the core converter.

• ical_to_jscal: extend per-child-VEVENT patch to include participants, locations, virtualLocations, alerts, status, keywords, freeBusyStatus
• ical_to_jscal: only include properties that differ from the master event
• ical_to_jscal: handle properties explicitly removed in an override instance
• jscal_to_ical: apply full patch dict to child VEVENT, not just title/start/duration/description
• jscal_to_ical: handle null patch values — remove the property from the child VEVENT
• Tests: round-trip recurring events with per-instance participant changes, location changes, alert changes, and combined multi-property overrides