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>

tools/docgen/inventory.json

@@ -420,5 +420,14 @@
     "uuid-to_text",
     "uuid-equals",
     "uuid-nil"
+  ],
+  "noise": [
+    "noise-value2",
+    "noise-perlin2",
+    "noise-simplex2",
+    "noise-fbm2",
+    "noise-cellular2",
+    "noise-cellular2_id",
+    "noise-unit"
   ]
 }

tools/x/selfhost.ludic

@@ -30,6 +30,7 @@ function selfhost_frags() -> []pointer {
   push(f, "selfhost/emit_hash.ludic")
   push(f, "selfhost/emit_crypto.ludic")
   push(f, "selfhost/emit_uuid.ludic")
+  push(f, "selfhost/emit_noise.ludic")
   push(f, "selfhost/emit_list.ludic")
   push(f, "selfhost/emit_ease.ludic")
   push(f, "selfhost/emit_collide.ludic")

tools/x/test.ludic

@@ -101,6 +101,7 @@ function cmd_test() -> int {
   # against known-answer vectors (crypto) or structural invariants (uuid/noise).
   feat_case("library/crypto", "", "1 2 3 4 5 6 7 8 9", "crypto.ludic (Crypto SHA-256/HMAC/base64 KAT + CSPRNG shape)")
   feat_case("library/uuid", "", "1 2 3 4 5 6 7 8 9 10", "uuid.ludic (Uuid v4/v7 format, version/variant, parse/equals)")
+  feat_case("library/noise", "", "1 2 3 4 5 6 7 8 9 10 11", "noise.ludic (Noise value/perlin/simplex/fbm/cellular determinism + range)")
 
   # issue #9: the Time/Date/Duration/Clock stdlib, driven from its own `entry`.
   net_case("lang/offline_rewards", "13 650 2026-08-30 0")