events: MONTHLY RRULE expansion (bymonthday + ordinal byday) + 13 tests

BYMONTHDAY (negative = from end), ordinal BYDAY ({:ord :wd}, last-weekday),
default day-of-month skipping short months. Weekly+monthly share ev-emit-occs.
37/37 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` → **24/24** (Phase 1: calendar recurrence)
+`bash lib/events/conformance.sh` → **37/37** (Phase 1: calendar recurrence)
 
 ## Ground rules
 
@@ -55,7 +55,7 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Phase 1 — Calendar + recurrence
 - [x] `calendar.sx` — event facts, RRULE expansion in a window (DAILY/WEEKLY)
-- [ ] `calendar.sx` — MONTHLY (bymonthday + nth-weekday byday)
+- [x] `calendar.sx` — MONTHLY (bymonthday + nth-weekday byday)
 - [ ] `availability.sx` — free/busy rules
 - [ ] `api.sx` + tests + scoreboard + conformance.sh
 
@@ -77,6 +77,11 @@ lib/events/api.sx ── (events/schedule) (events/book) (events/agenda) ──
 
 ## Progress log
 
+- 2026-06-06 — MONTHLY recurrence. `ev-days-in-month`, `ev-add-months`,
+  BYMONTHDAY (incl. negative = from month end), ordinal BYDAY (`{:ord N :wd W}`,
+  ord<0 = nth-from-last), default day-of-month (skips months too short, e.g.
+  day-31 monthly skips Feb/Apr). Refactored weekly+monthly onto a shared
+  `ev-emit-occs` per-period emitter. 37/37 green (+13).
 - 2026-06-06 — Phase 1 scaffold + calendar recurrence. `calendar.sx`: integer
   epoch-minute datetimes, Hinnant civil<->day-number conversion, DAILY/WEEKLY
   RRULE expansion in a bounded (start,end) window with INTERVAL, COUNT (window-