feat(stdlib): add Noise.* — deterministic fixed-point procedural noise (#3)

A Noise.* namespace for procedural generation, implemented entirely in Q16.16
fixed point over an integer permutation hash so a seed reproduces the exact same
field on every platform and run (native/headless/wasm) — the determinism edge
over float noise that drifts across CPUs.

  - value2 / perlin2 / simplex2  — value, gradient, and simplex noise -> [-1,1]
  - fbm2(x,y,seed,octaves)       — fractal Brownian motion (octaves of simplex)
  - cellular2 / cellular2_id     — Worley F1 distance + nearest-cell id
  - unit(n)                      — remap [-1,1] -> [0,1]

Covers issue phases 1–2 fully plus cellular from phase 3; domain warp, ridged/
billow, and sample1/sample3 remain as follow-ups. Pure integer IR, C-free;
cellular/fbm reuse the math prelude's fx_sqrt.

- examples/library/noise.ludic: asserts the invariants a fixed-point generator
  must hold (Perlin == 0 at lattice points, every sampler within [-1,1],
  reproducibility, seed sensitivity, non-negative cellular distance). Wired into
  `x test` (now 52 passed).
- docs: a new Noise 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

@@ -106,6 +106,7 @@ function emit_program() -> void {
   if g_uses_hashrt { emit_hash_prelude() }       # @fn_hash_fnv1a / @fn_hash_crc32 byte hashers
   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_datert { emit_datetime_prelude() }   # @fn_days_from_civil / @fn_civil_from_days conversions
 }