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>

tools/experiments/slope-flow.js

@@ -0,0 +1,29 @@
+// Reproducible generous-feed throughput and settled pile angles for t42.
+const { load, flatWorld } = require(process.env.DG_HARNESS || '../../test/harness');
+const { run } = load();
+const assert = require('node:assert/strict');
+for (const material of ['SAND','GRAVEL','CUORE','COAL']) {
+  const rates=[];
+  for (const grade of [0,-1]) {
+    flatWorld(run,256,224,208);
+    run(`RS=7;money=1e6;AMB.fill(20);temp.fill(20);hot.fill(0);
+      for(let k=0;k<10;k++)placeBlock(${grade?'BP_BELT_UP':'BP_BELT'},1,16+k,${grade?32:24}+k*${grade},true);
+      for(let x=10;x<16;x++)placeBlock(BP_BELT,1,x,${grade?33:24},true);
+      for(let x=26;x<32;x++)placeBlock(BP_BELT,1,x,${grade?23:24},true);`);
+    const result=run(`(()=>{let out=0;for(let k=0;k<1420;k++){
+      for(let l=1;l<=4;l++){const i=(${grade?132:96}-l)*W+44;if(mat[i]===EMPTY)setCell(i,${material},20,0);}
+      simTick();
+      for(let i=0;i<N;i++)if(mat[i]===${material} && i%W>=128){if(k>=220)out++;setCell(i,EMPTY,20,0);}
+      for(let j=flyers.length-1;j>=0;j--)if(flyers[j].m===${material} && flyers[j].x>=128){if(k>=220)out++;flyers.splice(j,1);}
+    }return out/20;})()`);
+    rates.push(result);
+  }
+  flatWorld(run,256,224,208);
+  run(`RS=7;AMB.fill(20);temp.fill(20);hot.fill(0);
+    for(let y=120;y<160;y++)for(let x=118;x<138;x++)setCell(y*W+x,${material},20,0);
+    for(let k=0;k<1200;k++)simTick();`);
+  // Apparent settled angle: heap height divided by half its footprint width.
+  const angle=run(`(()=>{let top=H,left=W,right=0;for(let i=0;i<N;i++)if(mat[i]===${material}){top=Math.min(top,(i/W)|0);left=Math.min(left,i%W);right=Math.max(right,i%W);}return Math.atan2(208-top,(right-left+1)/2)*180/Math.PI;})()`);
+  assert(rates[1]>=60 && rates[1]<=rates[0]*1.1, `${material} slope rate outside the target`);
+  console.log(`${material}: flat ${rates[0].toFixed(1)}, uphill ${rates[1].toFixed(1)} grains/s; pile angle ${angle.toFixed(1)} degrees`);
+}