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/emit_expr.ludic

@@ -247,6 +247,10 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
     if is_datetime_ns(meth) { return emit_datetime_ns(meth, e) }
     perr(`unknown builtin DateTime.{meth}`)
   }
+  if (ns == "Clock") {
+    if is_clock_ns(meth) { return emit_clock_ns(meth, e) }
+    perr(`unknown builtin Clock.{meth}`)
+  }
   var bare: pointer = null
   let labels = new []pointer
   if (ns == "Screen") {