Support recurring occurrences in aggregate GatherPress calendar feeds #1872

Problem

The recurring-events extension introduced in #1845 projects individual occurrences, but aggregate GatherPress calendar feeds remain series-based.

WordPressdotorg\GatherPress_Recurring_Events\Query::clauses() skips queries using fields => 'ids'. GatherPress' Event\Query::get_events_list() uses that mode, and Calendar\Setup uses the method for site-wide, event archive, venue, and taxonomy ICS feeds. Those feeds therefore include only the series' master event and can omit a recurring series once its master date is past, even when projected future occurrences remain.

Individual occurrence calendar links are already occurrence-aware. The wp-admin events list should remain series-based because projected occurrences are not independently editable posts.

Why this needs dedicated work

Removing the ID-query guard alone would return repeated copies of the same post ID without carrying the occurrence-specific date, URL, or calendar identity. A correct implementation needs an agreed ICS representation and the appropriate GatherPress extension points.

GatherPress/gatherpress#1684 already tracks the upstream RRULE/RDATE design discussion. This issue tracks applying that design to the experimental extension and supporting its aggregate feeds.

Expected outcome

• Choose an aggregate-feed representation aligned with Reserve ICS shape for future RRULE support when recurring events land GatherPress/gatherpress#1684, including cancellation or exception behavior.
• Add the minimum GatherPress core extensibility needed, or an extension-owned feed path if that is the better boundary.
• Include recurring occurrences correctly in site-wide, event archive, venue, and taxonomy ICS feeds.
• Preserve correct dates, URLs, and UID semantics in calendar clients.
• Add regression coverage for a recurring series whose master occurrence is already past but has future occurrences.

Current MVP behavior

The limitation is documented in #1845 and in plugins/gatherpress-recurring-events/README.md.