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

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

src/ui/threads.js

@@ -0,0 +1,39 @@
+// Browser transport only; the simulation owns the schedule and shared state.
+let threadStarting = null;
+async function startThreads() {
+  const count = Math.max(1, Math.min(16, Math.floor(SET.workerCap) || 16, (navigator.hardwareConcurrency || 3) - 2));
+  if (simPool && simPool.world === mat && simPool.workers.length === count) return;
+  if (threadStarting) await threadStarting;
+  if (simPool && simPool.world === mat && simPool.workers.length === count) return;
+  threadStarting = (async () => {
+    if (simPool) for (const worker of simPool.workers) worker.terminate();
+    simPool = null;
+    if (!(mat.buffer instanceof SharedArrayBuffer)) throw Error('Shared memory is unavailable');
+    const url = URL.createObjectURL(new Blob([SIM_WORKER_SOURCE, '\nonmessage=e=>threadMessage(e.data);'], {type:'text/javascript'}));
+    const pool = { world: mat, workers: [], slots: [] };
+    try {
+      const ready = [];
+      for (let k = 0; k < count; k++) {
+        const worker = new Worker(url), slot = threadSlot();
+        pool.workers.push(worker); pool.slots.push(slot);
+        ready.push(new Promise((resolve,reject) => {
+          const deadline = performance.now() + 10000;
+          worker.onerror = e => {
+            const error = Error(e.message || 'Simulation worker failed');
+            threadFailure(slot, error); reject(error);
+          };
+          const poll = () => {
+            if (Atomics.load(slot.control,0)) resolve();
+            else if (performance.now() > deadline) reject(Error('Simulation worker startup timed out'));
+            else setTimeout(poll, 1);
+          };
+          poll();
+        }));
+        worker.postMessage({world:threadWorld(),slot});
+      }
+      await Promise.all(ready); simPool = pool;
+    } catch (e) { for (const worker of pool.workers) worker.terminate(); throw e; }
+    finally { URL.revokeObjectURL(url); }
+  })();
+  try { await threadStarting; } finally { threadStarting = null; }
+}