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>

lib/events/scoreboard.json

@@ -1,10 +1,10 @@
 {
   "lang": "events",
-  "total_passed": 24,
+  "total_passed": 37,
   "total_failed": 0,
-  "total": 24,
+  "total": 37,
   "suites": [
-    {"name":"calendar","passed":24,"failed":0,"total":24}
+    {"name":"calendar","passed":37,"failed":0,"total":37}
   ],
-  "generated": "2026-06-06T23:48:10+00:00"
+  "generated": "2026-06-06T23:52:14+00:00"
 }