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/docgen/inventory.json

@@ -315,7 +315,8 @@
     "time-frame",
     "time-delta",
     "time-elapsed",
-    "time-now"
+    "time-now",
+    "time-since"
   ],
   "types": [
     "type-bool",
@@ -366,5 +367,39 @@
     "vector-rotate",
     "vector-angle",
     "vector-from_angle"
+  ],
+  "duration": [
+    "duration-seconds",
+    "duration-minutes",
+    "duration-hours",
+    "duration-days",
+    "duration-as_seconds",
+    "duration-as_minutes",
+    "duration-as_hours",
+    "duration-as_days"
+  ],
+  "date": [
+    "date-new",
+    "date-year",
+    "date-month",
+    "date-day",
+    "date-weekday",
+    "date-is_leap",
+    "date-days_in_month",
+    "date-to_epoch",
+    "date-add_days",
+    "date-diff_days"
+  ],
+  "datetime": [
+    "datetime-from",
+    "datetime-date",
+    "datetime-add",
+    "datetime-year",
+    "datetime-month",
+    "datetime-day",
+    "datetime-weekday",
+    "datetime-hour",
+    "datetime-minute",
+    "datetime-second"
   ]
 }