sync: update from private repo (0cddeaa)

src/engine/async-mutex.ts

@@ -0,0 +1,16 @@
+/**
+ * Minimal async mutex: serializes async sections via a promise chain. Each
+ * runExclusive call waits for the previous one to settle before starting, so
+ * critical sections never interleave across awaits. Rejections are isolated —
+ * a failing section does not break the chain for later callers.
+ */
+export class AsyncMutex {
+  private tail: Promise<unknown> = Promise.resolve();
+
+  runExclusive<T>(fn: () => Promise<T>): Promise<T> {
+    const result = this.tail.then(() => fn());
+    // Keep the chain alive regardless of this section's outcome.
+    this.tail = result.then(() => undefined, () => undefined);
+    return result;
+  }
+}