iCalendar import: group RECURRENCE-ID overrides by UID

[eikopf]

Context

Per draft-ietf-calext-jscalendar-icalendar-22, when multiple VEVENT (or VTODO) components share the same UID, the one without RECURRENCE-ID is the base recurrence and those with RECURRENCE-ID are overrides. The overrides should be merged into the base object's recurrenceOverrides property as PatchObjects keyed by the RECURRENCE-ID value.

Current behaviour

Each VEVENT/VTODO component is translated independently into a separate entry record, regardless of UID. A recurring event with three overrides produces four independent entries rather than one entry with an recurrenceOverrides map.

Expected behaviour

During iCalendar import, components sharing a UID should be grouped:

• The component without RECURRENCE-ID becomes the base entry
• Components with RECURRENCE-ID become entries in the base's recurrence_overrides map, keyed by the recurrence-id datetime value
• The override entries should contain only the properties that differ from the base (PatchObject semantics), or at minimum the full override record