Faces cut into loose piles collapse outward instead of draining column by column (t93)

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

test/pile-cut.js

@@ -0,0 +1,34 @@
+// Shared Node/browser reproduction: vacuum the foot of settled 60-grain heaps, one stroke per tick.
+function pileCutWorld() {
+  alloc(768, 600); worldSeed = 7; RS = 123;
+  for (let x = 0; x < W; x++) { surfY[x] = 224; dirtY[x] = 234; graniteY[x] = basaltY[x] = magmaY[x] = H; }
+  computeAmbient(); fillNoise();
+  for (let y = 0; y < H; y++) for (let x = 0; x < W; x++) {
+    const i = y * W + x; mat[i] = y >= 224 ? STONE : EMPTY;
+    flags[i] = y >= 224 ? F_NAT : 0; temp[i] = AMB[y];
+  }
+  bag.fill(0); bagTotal = 0; player.tankTier = 3;
+  resetWeather(); weather.next = 1e15; updateRoof();
+}
+function pileCutSetup(peak, mixed) {
+  const rubble = [RSTONE, CUORE, FEORE, LDIRT];
+  for (let y = 164; y < 224; y++) for (let x = peak - (y - 164); x <= peak + (y - 164); x++)
+    setCell(y * W + x, mixed ? rubble[(x + y) % rubble.length] : RSTONE, AMB[y], 0);
+}
+function pileCutStroke(peak) {
+  bot.x = peak - 50; bot.y = 170; bot.vx = bot.vy = 0;
+  for (let y = 223; y >= 164; y--) for (let x = peak - 100; x < peak; x++) {
+    if (PH[mat[y * W + x]] !== PH_POWDER) continue;
+    return toolVacuum(Math.min(peak - 7, x), y - 3, 6, () => true).got;
+  }
+  return 0;
+}
+function pileCutProfile(peak) {
+  let cliff = 0, previous = 0, grains = 0;
+  for (let x = peak - 110; x <= peak + 110; x++) {
+    let y = 0; while (y < 224 && PH[mat[y * W + x]] !== PH_POWDER) y++;
+    const height = 224 - y; cliff = Math.max(cliff, Math.abs(height - previous)); previous = height;
+    for (; y < 224; y++) if (PH[mat[y * W + x]] === PH_POWDER) grains++;
+  }
+  return {cliff, grains};
+}