Batch acquire: take up to N tasks per call across non-empty queues

先阅读 threadmill/backends/redis.py 和 acquire.lua，然后跟踪 WorkerProcess 和 WorkerThread，以了解当前按线程执行的 acquire 流程。定义 fetch-and-dispatch 如何处理 lease 计时、优雅关闭和预取任务，同时将 pop 和 move 保持在一次 script 调用中。完成标准是：可以从非空队列中获取最多 N 个任务，且不会有工作被搁置或过早失败。

Split out of #48 during the fair multi-queue scheduling change (codingjoe-fair-multi-queue-scheduling).

defer: Batch acquire (fetch up to N tasks per call across non-empty queues). Needs executor fetch-and-dispatch; the lease would tick while tasks wait in-process. [threadmill/backends/redis.py]

Why

acquire() pops a single task per script call, so a process with --threads 4 performs four Redis round trips to fill its threads. A batch acquire that spreads up to N tasks across the non-empty queues would cut round trips and drain several queues in parallel.

What makes it non-trivial

• WorkerProcess runs one WorkerThread per thread, each calling acquire() itself. A batch would need a fetch-and-dispatch step, since a single thread cannot execute N tasks at once.
• acquire.lua moves the task into the running set with deadline = now + lease_ttl. Tasks prefetched into an in-process queue would burn lease budget while waiting, so the reaper could fail them, or the lease clock has to start at execution instead.
• Graceful shutdown must return prefetched, unstarted tasks to the ready set instead of leaving them to be reaped as FAILED.

Pop and move must stay inside one script call.