events: waitlist + auto-promotion + 21 tests

When full, ev/waitlist! queues actors FIFO (:waitlist/:unwaitlist on the
booking stream; waiting fold independent of the seat fold). ev/waitlist,
ev/waitlist-position, ev/leave-waitlist!. ev/cancel-promote! frees a seat and
auto-promotes the head of the queue to a confirmed booking. Idempotent.
240/240 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/events-on-sx.md

@@ -18,7 +18,7 @@ capacity rules, transactional booking, and a flow-driven notification dispatcher
 
 ## Status (rolling)
 
-`bash lib/events/conformance.sh` → **219/219** (Phases 1-4 + ext: federated free/busy)
+`bash lib/events/conformance.sh` → **240/240** (Phases 1-4 + ext: federated free/busy, waitlist)
 
 ## Ground rules
 
@@ -86,6 +86,13 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-07 — Waitlist + auto-promotion (extension). When an occurrence is
+  full, `ev/waitlist!` queues actors FIFO (:waitlist/:unwaitlist events on the
+  same stream; waiting fold is independent of the seat fold since taking a seat
+  removes from the queue). `ev/waitlist` (queue), `ev/waitlist-position`,
+  `ev/leave-waitlist!`. `ev/cancel-promote!` cancels a seat and auto-promotes
+  the head of the queue to a confirmed booking when capacity opens. Idempotent
+  (:already / :already-waiting). +21 tests, 240/240 green.
 - 2026-06-07 — Federated free/busy (extension). Peers publish BUSY intervals
   per actor (iCal free/busy model — privacy-preserving, not event details).
   `ev/peer-with-busy`, `ev/peer-busy`; `ev/federated-busy` unions local