events: capacity-safe transactional booking on persist + 24 tests

booking.sx: per-occurrence append-only stream, roster = replay. Booking
decided against an observed (roster, last-seq) snapshot, committed via
persist/append-expect — atomic check+append, no overbooking, no lock.
Explicit last-seat race test: two bookers, one booked, one conflict, roster
capped. Idempotent per actor. 97/97 green.

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

lib/events/scoreboard.json

@@ -1,12 +1,13 @@
 {
   "lang": "events",
-  "total_passed": 73,
+  "total_passed": 97,
   "total_failed": 0,
-  "total": 73,
+  "total": 97,
   "suites": [
     {"name":"calendar","passed":37,"failed":0,"total":37},
     {"name":"availability","passed":22,"failed":0,"total":22},
-    {"name":"api","passed":14,"failed":0,"total":14}
+    {"name":"api","passed":14,"failed":0,"total":14},
+    {"name":"booking","passed":24,"failed":0,"total":24}
   ],
-  "generated": "2026-06-07T01:15:49+00:00"
+  "generated": "2026-06-07T01:44:19+00:00"
 }