Calendar V5: events that repeat by rule — every full moon, the 3rd Kingsday, two days after a festival #766

Why this issue exists: the 2026-07-23 calendar brief promised a "condition-recurrence builder" ("every full moon of Luna", "3rd Kingsday of each month", "2 days after Festival of Masks"). The Calendaria parity audit ranked the lack of it the #2 gap: repeats were a four-value list, so a GM had to create each such occurrence by hand. It isn't in #741's checklist, which only covers simple repeats (interval, end, count) for Foundry.

What it is: when an event repeats, the Director can choose a rule as well as "every N days/weeks/months/years":

• tied to a moon's phase ("every full moon of Luna");
• tied to the calendar ("the 3rd Kingsday of each month", "the last day of the year");
• tied to another event or festival ("2 days after the Festival of Masks").

Each occurrence can still be moved or skipped on its own.

Watch for:

• Occurrences need their own identity, because RSVPs are per occurrence (see Calendar V5: requirements and lessons carried over from the v4 calendar #741).
• Rules must use the calendar's single day count, so they never drift from the grid (the lesson Calendar V5: requirements and lessons carried over from the v4 calendar #741 records).
• Round-trip with Calendaria where it can express the rule. Where it can't, sync the occurrences it produces.

When: with or after V5's events, before generated events build on it.