Screw tubes carry bars, one whole bar at a time

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

src/sim/conveyors.js

@@ -106,6 +106,7 @@ function tubeSpread(c, x, y, sx, sy, intake = false) {
 // An empty screw cell draws in loose material from behind it or from its sides
 // (not from the cell it pushes into), so a tube end sitting in a pile feeds itself.
 function tubeIntake(i, p, dx, dy) {
+  if (occ[i]) return;
   const x = i % W, y = (i / W) | 0;
   const cands = [[-dx, -dy], [dy, dx], [-dy, -dx]];
   const start = (tick + i) % 3;