Background scene: sky costs no noise; one sun term

Sky rays were ~90% of pass 1 (noSkyMarch variant: 292 -> 26 ms, SwiftShader 960x540 L0).
terrainDistance now returns early where the plateau mask is 0 (height exactly 0), skipping
fbm and Phacelle there: pass 1 292 -> 159 ms, same step values. skyColor keeps one sun term
(the wide glow is gone; screenshots differ by that and by TAA/time, not compared pixelwise).
Harness: noSkyMarch variant, ONLY=<names> selects variants.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

harness/scene/drive.cjs

@@ -90,9 +90,11 @@ const [, , dir, mode, outdir = '.'] = process.argv;
         'float terrainFbm(vec2 xz) { float a = 1.0, fr = 1.0, v = 0.0; for (int i = 0; i < W_OCTAVES; i++) { v += a * cultmath_snoise(vec3(xz * W_FREQ * fr, 0.0)); fr *= W_LACUNARITY; a *= W_GAIN; } return v; }',
       ],
       noErosion: ['#define W_ERODE_ENABLED 1', '#define W_ERODE_ENABLED 0'],
+      // Upper bound on what skipping sky rays would save: every upward ray misses at once.
+      noSkyMarch: ['    bool hit = false;\n    bool onTree = false;', '    if (rd.y > 0.0) { o_color = vec4(0.0); o_depth = W_FAR; return; }\n    bool hit = false;\n    bool onTree = false;'],
       noTree: ['float dr = treeDistance(p, leafness);', 'float dr = 1e9; leafness = 0.0;'],
     };
-    const wanted = mode === 'variants' ? Object.keys(VARIANTS) : ['baseline'];
+    const wanted = mode === "variants" ? (process.env.ONLY ? process.env.ONLY.split(",") : Object.keys(VARIANTS)) : ['baseline'];
     const sizes = [
       [960, 540],
       [540, 1200],

src/app/features/scene/shaders/common.glsl

@@ -26,6 +26,6 @@ vec3 skyColor(vec3 rd)
     float h = clamp(rd.y, 0.0, 1.0);
     vec3 c = mix(u_skyHorizon, u_skyTop, pow(h, 0.45));
     float s = max(dot(rd, u_sunDir), 0.0);
-    c += u_sunColor * (0.25 * pow(s, 8.0) + 1.5 * pow(s, 256.0));
+    c += u_sunColor * (1.5 * pow(s, 256.0));
     return c;
 }

src/app/features/scene/shaders/terrain.frag.glsl

@@ -30,6 +30,10 @@ CultPhasor terrainPhacelle(vec2 xz)
 float terrainDistance(vec3 p)
 {
     float m = terrainMask(p.xz);
+    // Flat plateau: the height is exactly 0, so neither noise is evaluated. Sky rays spend most
+    // of their steps here.
+    if (m <= 0.0)
+        return p.y * W_SLOPE_ENV_INV;
     float hb = m * W_AMP * terrainFbm(p.xz);
     float above = p.y - (hb + m * W_ERODE_AMP);
     if (above > 0.0)