perf(redis): wake fewer workers per job #35

Problem

With #34, an idle Redis worker waits on a marker with BZPOPMIN instead of polling. When a single Redis core saturates, the PR drains fewer jobs than polling did, because each job costs more round trips.

Measured on a 16-vCPU orb, Redis 7.4 pinned to one dedicated core, workers and producer pinned to other cores, 6 interleaved reps per variant (head 9b55a64):

Redis time per job: 102 µs on main, 138 µs on the PR. Round trips cost Redis about 14 µs each, more than the work inside the scripts.

Where the extra round trips go

A job wakes several workers. Per job, at saturation:

• 1.2 to 1.9 BZPOPMIN returns;
• 1.3 to 2.4 popFrom calls that find nothing;
• up to 0.34 RELEASE_MARKER calls (32 workers, concurrency 10).

The push adds a ready marker, and the acquisition adds it again while jobs remain, so both often wake a worker for the same job.

Ideas, untested

• Re-mark only when more jobs remain than workers already woken, or carry a count in the marker.
• Let the acquisition script pop the next job for a worker woken by a marker, so a wake costs one round trip.
• Drop the release round trip for a ready marker popped after the wait ended, if another marker already covers the queue.

Any change must keep the guarantee that a stored job wakes at least one idle worker, or is found at the next stalled check.