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_id.md

@@ -0,0 +1,31 @@
+---
+id: noise-cellular2_id
+name: Noise.cellular2_id
+category: noise
+kind: namespace-method
+tokens: Noise.cellular2_id
+sig: Noise.cellular2_id(x, y, seed) -> int
+tip: The id of the nearest Worley cell — stable per cell.
+order: 6
+ns: Noise
+member: cellular2_id
+---
+
+Returns the integer **id of the nearest cell** in the same Worley diagram that <a href="noise-cellular2"><code>Noise.cellular2</code></a> measures distance in — a stable hash that is identical for every sample point inside a given cell. Use it to assign something discrete per region: the biome or material of a cell, the variant of a scattered prop, a per-region colour. Combine it with the F1 distance to shade toward cell edges.
+
+The id is a hash, so treat it as an opaque label (mod it into your table of choices); it is deterministic across platforms and runs.
+
+Parameters:
+- `x`, `y` — the sample coordinates (`fixed`)
+- `seed` — the field selector (must match the `cellular2` call it pairs with)
+
+```ludic
+program CellId {
+  entry {
+    let seed = 7
+    let id = Noise.cellular2_id(fixed(5) / fixed(4), fixed(3) / fixed(4), seed)
+    let biome = Math.posmod(id, 4)   # one of 4 biomes for this cell
+    print(biome)
+  }
+}
+```