events: next-free slot search + 6 tests

ev-next-free finds the earliest free slot >= after for a duration within a
horizon, probing 'after' + busy-interval ends via the busy_in rule (ev-free?).
Finds gaps, skips too-short gaps, half-open at edges. 59/59 green.

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

lib/events/scoreboard.json

@@ -1,11 +1,11 @@
 {
   "lang": "events",
-  "total_passed": 53,
+  "total_passed": 59,
   "total_failed": 0,
-  "total": 53,
+  "total": 59,
   "suites": [
     {"name":"calendar","passed":37,"failed":0,"total":37},
-    {"name":"availability","passed":16,"failed":0,"total":16}
+    {"name":"availability","passed":22,"failed":0,"total":22}
   ],
-  "generated": "2026-06-07T00:21:06+00:00"
+  "generated": "2026-06-07T00:49:23+00:00"
 }