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>

docs/language/clock/clock-set.md

@@ -0,0 +1,22 @@
+---
+id: clock-set
+name: Clock.set
+category: clock
+kind: namespace-method
+tokens: Clock.set
+sig: Clock.set(t)
+tip: Set the clock to an instant.
+order: 1
+ns: Clock
+member: set
+---
+
+Sets the simulated clock to the instant <code>t</code>. Typically called once on load (e.g. from a saved <code>Time.now()</code>), after which gameplay advances it deterministically.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    Clock.set(DateTime.from(2026, 8, 30, 8, 0, 0))
+  }
+}
+```