Sidewall belt and screen drawing, cheaper cleat checks (t42, t43)

Far sidewall drawn behind the load with cleat ribs at their physics
positions; sloped screens get a tinted mesh between rails and shake only
when loaded. Powder skips cleat checks outside chunks that have cleats,
and empty slopes let their chunks sleep. Record the screw conveyor and
stall/carry-depth decisions.

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

src/render/slope-belt-shader.js

@@ -39,6 +39,22 @@ vec3 slopeShape(vec2 p, vec2 worldOrigin, float grade, int joins) {
 `;
 
 const SM_SLOPE_DRAW = `
+  if (nc == 31 || nc == 32) {
+    vec4 tag = cellAt(tTree, c);
+    float grade = nc == 31 ? -1.0 : 1.0;
+    vec2 local = floor(tag.gb * 255.0 + 0.5) - 8.0 + fract(w);
+    float sy = grade < 0.0 ? 4.0 : 0.0;
+    float foot = (local.x + grade * (local.y - sy)) * 0.5;
+    float depth = (grade * local.x - local.y + sy) * 0.70710678;
+    int wallFlags = int(tag.a * 255.0 + 0.5);
+    float ends = max((wallFlags & 8) == 0 ? -foot : -100.0, (wallFlags & 16) == 0 ? foot - 4.0 : -100.0);
+    float sd = max(ends * 1.41421356, max(-depth, depth - ${CLEAT_HEIGHT.toFixed(1)}));
+    float phase = float(wallFlags & 7);
+    float wave = 0.5 + 0.5 * sin((foot + w.x - local.x - phase) * 3.14159265);
+    vec3 rubber = mix(vec3(0.16,0.18,0.21), vec3(0.21,0.23,0.27), wave);
+    C = mix(C, rubber, 1.0 - smoothstep(-px, px, sd));
+  }
+
   // Slopes use the flat belt's band, rollers and mesh in coordinates along the run.
   if ((nc == 2 || (nc >= 23 && nc <= 30)) &&
       (int(cellAt(tTree, c).a * 255.0 + 0.5) & 128) != 0 &&
@@ -58,9 +74,7 @@ const SM_SLOPE_DRAW = `
     float rg = step(0.72, fract((axis - dir * (depth < 2.0 ? 1.0 : -1.0) * time * 30.0) * 0.5));
     vec3 belt;
     if (-sd < (mesh ? 1.1 : 0.8)) {
-      belt = bandColor(lb, rg);
-      if (mesh && ((joins & 32) != 0) == (depth < 2.0) && dot(own.rgb, vec3(1)) > 0.01)
-        belt = mix(belt, own.rgb * (depth < 2.0 ? 1.0 : 0.8), 0.75 * rg);
+      belt = mesh ? mix(vec3(0.22,0.25,0.29), vec3(0.48,0.53,0.59), depth < 2.0 ? 0.85 : 0.35) : bandColor(lb, rg);
       belt *= mix(0.55, 1.0, smoothstep(0.0, max(px * 1.4, 0.12), -sd));
     } else if (mesh) {
       vec2 cg = abs(fract(alongWorld) - 0.5);