events: Phase 1 calendar — DAILY/WEEKLY RRULE expansion + 24 tests

Civil date arithmetic (Hinnant), integer epoch-minute datetimes, bounded
windowed RRULE expansion (DAILY/WEEKLY with INTERVAL/COUNT/UNTIL/BYDAY),
multi-event merge. Conformance harness + scoreboard wired.

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

lib/events/scoreboard.json

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