feat(stdlib): DateTime format/parse + simulated Clock — completes #9

Finish issue #9 by adding the two remaining acceptance items on top of the
calendar/clock core, still pure-integer and deterministic:

  DateTime.format(dt, pattern) -> string   render an instant via a token
                                           pattern (YYYY/YY/MM/DD/HH/mm/ss;
                                           other chars pass through)
  DateTime.parse(text, pattern) -> int     read an instant back; -1 on a
                                           non-digit where one is expected
  Clock.now/set/advance/reset              a game-controlled simulated clock
                                           (the @L_clock global) that never
                                           touches the wall clock, so gameplay
                                           reading Clock.now() is replay-safe

format/parse take a string-LITERAL pattern and are expanded at compile time
(field offsets are then constant), folding @fn_str_concat over literal runs and
two small runtime helpers: @fn_dt_pad0 (zero-padded field) and @fn_dt_rd
(fixed-width digit reader that stops at the terminator and flags malformed
input). Clock is a universal i32 global declared in emit_head, so it works in
entry and game programs alike.

Adds examples/offline_rewards.ludic — the issue's worked "you were away N hours"
example, driven from its own entry and asserted in the regression suite — plus
selfhost/tests/datetime2.ludic (format/parse round-trip, parse failure, clock),
docs (Clock section + 4 pages, DateTime.format/parse pages), inventory and LSP
hover. Reseeded; C-free fixpoint holds; all suites green (27 self-host / 46
regression / 29 tools); check.py (366 symbols), check-impl.py (218 ns-methods)
and validate.py OK.

With this, #9's scope is fully delivered: DateTime/Date/Duration + core ops,
format/parse, a deterministic simulated clock, docs + offline-rewards example,
and tests. (v1 stays UTC-only, no leap seconds, i32 epoch valid through 2038.)

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

selfhost/tests/datetime2.ludic

@@ -0,0 +1,23 @@
+program T {
+  entry {
+    let t = DateTime.from(2026, 8, 30, 7, 5, 9)
+
+    # format — tokens expand zero-padded, other chars pass through
+    print(DateTime.format(t, "YYYY-MM-DD"))   # 2026-08-30
+    print(DateTime.format(t, "HH:mm:ss"))     # 07:05:09
+    print(DateTime.format(t, "DD/MM/YY"))     # 30/08/26
+
+    # parse — round-trips to the same instant; missing fields default to midnight
+    print(DateTime.parse("2026-08-30", "YYYY-MM-DD") - Date.to_epoch(Date.new(2026, 8, 30))) # 0
+    print(DateTime.parse("zz", "YYYY"))       # -1 (non-digit -> parse fails)
+
+    # Clock — the game-controlled simulated clock
+    Clock.set(3600)
+    print(Clock.now())                        # 3600
+    Clock.advance(Duration.minutes(30))
+    print(Clock.now())                        # 5400
+    print(DateTime.minute(Clock.now()))       # 30
+    Clock.reset()
+    print(Clock.now())                        # 0
+  }
+}