events: provisional holds (hold/confirm/release) for paid tickets + 24 tests

Booking stream gains :hold/:confirm/:release; fold tracks per-actor seat state
(:held/:confirmed). A held seat counts toward capacity so a pending payment
can't be oversold. ev/hold! (capacity-safe), ev/confirm!, ev/release!,
ev/seat-state. Holds race test mirrors the booking race. 144/144 green.

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

lib/events/scoreboard.json

@@ -1,13 +1,13 @@
 {
   "lang": "events",
-  "total_passed": 120,
+  "total_passed": 144,
   "total_failed": 0,
-  "total": 120,
+  "total": 144,
   "suites": [
     {"name":"calendar","passed":37,"failed":0,"total":37},
     {"name":"availability","passed":22,"failed":0,"total":22},
     {"name":"api","passed":24,"failed":0,"total":24},
-    {"name":"booking","passed":37,"failed":0,"total":37}
+    {"name":"booking","passed":61,"failed":0,"total":61}
   ],
-  "generated": "2026-06-07T02:39:08+00:00"
+  "generated": "2026-06-07T03:07:09+00:00"
 }