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>

tools/x/selfhost_test.ludic

@@ -47,6 +47,7 @@ function cmd_selfhost_test() -> int {
   sh_case("math3", "45 90 180 -135 30 60 90")
   sh_case("transcend", "2718 1 148 1000 0 2303 1024 1414 3 250 0 1000 1016")
   sh_case("vector", "3 4 5 4 6 2 2 11 2 6 999 999 999 1570 5 10")
+  sh_case("datetime", "10800 330 172800 3 2 0 10957 2026 8 30 4 6 1 0 1 29 28 31 2 1 365 946729815 2000 1 1 12 30 15 6 10957 1")
   sh_case("textsplit", "1 1 1 3 1 1 1 1 1 1 1")
   sh_case("hash", "-2128831035 -468965076 114400290 114400290 0 -873187034 1095738169 0 1364076727 -2114883783 -845898438 -845898438 -78065325 -78057399 -3750763034362895579 -5808556873153909620 -4100651535478758590 -4100651535478758590 0 -5451962507482445012 7256831767414464289")
   sh_case("long", "1000000000000 1000000000001 1000000000005 3000000000000 1 1 1 -1000000000000 1000000 13")