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>

tools/x/selfhost_test.ludic

@@ -48,6 +48,7 @@ function cmd_selfhost_test() -> int {
   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("datetime2", "2026-08-30 07:05:09 30/08/26 0 -1 3600 5400 30 0")
   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")