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

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.