feat(stdlib): add Log.* — levelled, structured logging (#15)

A Log.* namespace: five levels (trace/debug/info/warn/error), a runtime
threshold, and structured key=value fields, so games get something better than
scattered print calls and release builds can go quiet without touching call
sites.

  - Log.trace/debug/info/warn/error(msg, [k, v]...)  -> stderr, "[LEVEL] msg k=v"
  - Log.set_level(n)   show only level >= n (0 = all default, 5 silences all)
  - Log.level()        read the current threshold

Fields accept strings, ints, and longs (numbers formatted automatically); the
level tag is chosen at compile time so a filtered-out level costs only a
comparison. Writes to stderr, never touching the simulation — no effect on
determinism/replays. v1 is the console sink; rotating-file and in-engine overlay
sinks are noted as follow-ups.

- examples/library/logging.ludic: asserts the set_level/level threshold
  round-trip and that every level (with mixed-type fields) runs without faulting;
  the stderr gating itself was verified by hand (warn/error emit, lower levels
  suppressed). Wired into `x test` (now 53 passed).
- docs: a new Log section + per-symbol pages; inventory and coverage pass.
- seed regenerated; `x bootstrap-cfree` fixpoint holds.

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

selfhost/emit_decl.ludic

@@ -107,6 +107,7 @@ function emit_program() -> void {
   if g_uses_cryptort { emit_crypto_prelude() }   # @fn_sha256_hex / @fn_hmac_sha256_hex + constant-time compare + CSPRNG
   if g_uses_uuidrt { emit_uuid_prelude() }       # @fn_uuid_v4 / @fn_uuid_v7 / parse / equals (over the crypto CSPRNG)
   if g_uses_noisert { emit_noise_prelude() }     # @fn_noise_value2/perlin2/simplex2/fbm2/cellular2 (Q16.16)
+  if g_uses_logrt { emit_log_prelude() }         # @L_log_level + @fn_log_emit (levelled stderr sink)
   if g_uses_datert { emit_datetime_prelude() }   # @fn_days_from_civil / @fn_civil_from_days conversions
 }