Multi-core simulation: grain movement and heat across a worker pool (t88 phase 3)

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

test/t_conveyors.js

@@ -123,7 +123,8 @@ const liftLanes = run('(() => { const s = new Set(); for (let y = 76; y < 80; y+
 check('a lift turns its corner without squeezing down to one lane', liftLanes === 4 && lr > 30, `${lr.toFixed(0)}/s, ${liftLanes} lanes at the top`);
 fresh();
 run('for (let bx = 20; bx < 32; bx++) placeBlock(BP_BELT, 1, bx, 25, true);');
-rect(run, 80, 20, 95, 99, run('SAND'));
+// A wide heap keeps the belt fed across the new chunk-local random streams.
+rect(run, 80, 20, 111, 99, run('SAND'));
 const br = rate(128, 319, 104, 119, 90, 300);
 ticks(run, 0);
 check('a belt carries a pile off at several dozen grains a second', br > 50, `${br.toFixed(0)}/s`);