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-unit.md

@@ -0,0 +1,27 @@
+---
+id: noise-unit
+name: Noise.unit
+category: noise
+kind: namespace-method
+tokens: Noise.unit
+sig: Noise.unit(n) -> fixed
+tip: Remap a [-1,1] noise sample to [0,1].
+order: 7
+ns: Noise
+member: unit
+---
+
+Remaps a noise sample from <code>[-1, 1]</code> to <code>[0, 1]</code> — literally <code>n / 2 + 0.5</code> — returning a <a href="type-fixed"><code>fixed</code></a>. The signed range is the natural output of the samplers, but heights, densities, probabilities, and colour ramps usually want the unsigned <code>[0, 1]</code> range; <code>unit</code> is the one-step conversion. It is a plain affine remap, so <code>unit(-1) == 0</code>, <code>unit(0) == 0.5</code>, and <code>unit(1) == 1</code>.
+
+Parameters:
+- `n` — a sample in `[-1, 1]` (e.g. from `perlin2`/`simplex2`/`fbm2`)
+
+```ludic
+program Unit {
+  entry {
+    let n = Noise.simplex2(fixed(1) / fixed(2), fixed(1) / fixed(2), 1337)
+    let height = Math.floor(Noise.unit(n) * fixed(64))   # 0..64 tiles
+    print(height)
+  }
+}
+```