fix(queue): survive a synchronous task throw in SerialQueue

[YOMXXX]

Fixes #518.

Problem

SerialQueue.drain() called entry.task() directly. If a queued task throws synchronously, the call escapes before the .finally() handler is registered — the promise returned by add() rejects, but the queue stays permanently stuck with running = true.

This affects all three pipeline queues (L1/L2/L3, pipeline-manager.ts). Subsequent tasks never start, and callers such as flushSession() and _doFlush() can wait forever on onIdle().

Reproduction (on main):

const queue = new SerialQueue("repro");
await queue.add(() => { throw new Error("sync failure"); }).catch(() => undefined);
const next = queue.add(async () => "ran");
// hangs forever — queue.running stays true

Change

Invoke the task from a resolved promise (Promise.resolve().then(() => entry.task())) so a synchronous throw is routed through the catch/finally chain and queue bookkeeping is finalized.

Verification

• New src/utils/serial-queue.test.ts (4 tests): sync-throw recovery, FIFO order across mixed tasks, onIdle() after a sync-throw, and no double-run.
• Confirmed the first test hangs on the unfixed code and passes after the fix.
• npm test + npm run build:plugin pass.

[Maxwell-Code07]

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