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>

docs/language/noise/noise-cellular2.md

@@ -0,0 +1,30 @@
+---
+id: noise-cellular2
+name: Noise.cellular2
+category: noise
+kind: namespace-method
+tokens: Noise.cellular2
+sig: Noise.cellular2(x, y, seed) -> fixed
+tip: Worley (cellular) F1 distance to the nearest cell point.
+order: 5
+ns: Noise
+member: cellular2
+---
+
+Samples **cellular (Worley) noise** at <code>(x, y)</code> and returns the F1 distance — the distance to the nearest feature point — as a <a href="type-fixed"><code>fixed</code></a> (roughly <code>[0, 1.5]</code>). Each lattice cell holds one feature point placed by its hash; scanning the 3×3 neighbourhood finds the closest one. The distance field forms Voronoi cells, which are exactly the structure you want for stone and cracked textures, biome or region boundaries, and scattered-feature layouts. Small distances mark cell centres; ridges appear where two cells meet.
+
+Pair it with <a href="noise-cellular2_id"><code>Noise.cellular2_id</code></a> to also know *which* cell you are in. Deterministic in fixed point across platforms and runs.
+
+Parameters:
+- `x`, `y` — the sample coordinates (`fixed`)
+- `seed` — the field selector
+
+```ludic
+program Cellular {
+  entry {
+    let seed = 7
+    let d = Noise.cellular2(fixed(5) / fixed(4), fixed(3) / fixed(4), seed)
+    print(Math.floor(d * fixed(1000)))   # distance to nearest cell point
+  }
+}
+```