feat(stdlib): Time/Date/Duration calendar-clock core (issue #9)

Implement the calendar/clock half of #9 as plain-i32 integer epochs — no
new type, no floating point (the issue's "integer epochs to avoid drift") —
so every operation is deterministic and bit-identical on every platform:

  Duration — a span in whole seconds; seconds/minutes/hours/days build one,
             as_seconds/as_minutes/as_hours/as_days read it back. Because a
             duration is just an int, `+` and `>` work with no extra machinery
             (Duration.minutes(5) + Duration.seconds(30), away > Duration.hours(3)).
  Date     — a civil day as days-since-1970 (UTC): new/year/month/day/weekday/
             is_leap/days_in_month/to_epoch/add_days/diff_days.
  DateTime — an instant as seconds-since-1970 (UTC, matching Time.now):
             from/date/add/year/month/day/weekday/hour/minute/second.
  Time.since(past) = now - past, for offline-progress / "time away" checks.

New selfhost/emit_datetime.ludic (is_/emit_ for the three namespaces, wired
into emit_ns_call + the frag list). The two civil<->epoch conversions are
Howard Hinnant's public-domain proleptic-Gregorian algorithms, emitted once
per program as the @fn_days_from_civil / @fn_civil_from_days prelude and gated
by g_uses_datert; days_in_month is next-month-day-0 (no lookup table). Time
gains `since`. Docs (Duration/Date/DateTime sections, 28 method pages +
time-since), inventory, and LSP hover kept in sync; a registered test checks
component math against hand-computed values. Reseeded; C-free fixpoint holds;
all suites green (26 self-host / 45 regression / 29 tools); check.py,
check-impl.py and validate.py OK.

format/parse, a game-controlled simulated clock, and timezones are tracked
follow-ups; v1 is UTC-only and, on the i32 epoch, valid through 2038.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/datetime/datetime-second.md

@@ -0,0 +1,22 @@
+---
+id: datetime-second
+name: DateTime.second
+category: datetime
+kind: namespace-method
+tokens: DateTime.second
+sig: DateTime.second(dt) -> int
+tip: The second of the minute (0-59) of an instant.
+order: 9
+ns: DateTime
+member: second
+---
+
+The second within the minute of the instant <code>dt</code> (UTC), <code>0</code>&ndash;<code>59</code>.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let s = DateTime.second(DateTime.from(2000, 1, 1, 12, 30, 15))   # 15
+  }
+}
+```