Sloped belts hold their load with moving cleats (t42); sloped filters are downhill shaking screens (t43)

The magic grip is gone: cleats every 8 face cells, 6 grains tall, carry
pockets uphill (about 71 grains/s against 90 flat) and overflow rolls
back. Sloped filters always run downhill; grains travel by gravity and
a shake, and sorting stays exact. Drawing needs another pass.

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

src/ui/previews.js

@@ -221,6 +221,20 @@ function previewBlock(code, x, y, w, h, stroke = false, direction = -1, bx = -1,
     }
     if (stroke) { const fill = ctx.fillStyle; ctx.fillStyle = ctx.strokeStyle; ctx.fill(); ctx.fillStyle = fill; }
     else ctx.fill();
+    // Preview cleats stand perpendicular to the face; screens have a raised far rail.
+    ctx.beginPath();
+    for (let k = 0; k < BS; k++) {
+      const fy = beltFaceY(code, 0, 0, k);
+      if (isFilter(code)) ctx.rect(x + k * scale, y + (fy - 2) * scale, scale, scale);
+      else {
+        ctx.rect(x + k * scale, y + (fy - 7 - ((ox + k) & 1)) * scale, scale, (2 + ((ox + k) & 1)) * scale);
+      }
+      if (!isFilter(code) && (((ox + k) % CLEAT_SPACING + CLEAT_SPACING) % CLEAT_SPACING === 0))
+        for (let h = 0; h <= CLEAT_REACH; h++)
+          ctx.rect(x + (k + beltGrade(code) * h) * scale, y + (fy - h - 1) * scale, scale, scale);
+    }
+    if (stroke) { const fill = ctx.fillStyle; ctx.fillStyle = ctx.strokeStyle; ctx.fill(); ctx.fillStyle = fill; }
+    else ctx.fill();
     return;
   }
   code = slopeShape(code);