perf(rollout): six independent rollout-path fixes and knobs

[Rockdu]

What

Six independent rollout-path improvements, one commit each:

Commit	Change
perf(rm_hub)	submit reward chunks to Ray inside the worker thread — .remote() serialises the payload into plasma in the calling thread, so submitting on the event loop stalled every concurrent request
perf(rm_hub)	select the reward frames before converting a clip to uint8 (8 of 107 frames survive; converting all first cost several full-size float32 copies)
fix(router)	register /remove_worker; engines call it on shutdown and it fell through the catch-all, proxied to a worker as a generate request
fix(fsdp)	report checkpoint key mismatches — the LoRA path loads with strict=False and discarded set_state_dict's result, so a checkpoint whose keys match nothing "loaded" silently
perf(rollout)	--rollout-request-stagger: one-shot jitter before slot acquisition; co-admitted requests stay in phase for the whole rollout (measured: median inter-arrival gap 0.00 s, 16 arrivals in the first 5% of each rollout)
feat(rollout)	--rollout-video-dtype {keep,uint8}: opt-in engine-side quantisation of the rollout video, default unchanged; needs the sglang counterpart to take effect and is harmless without it

Validation

• Frame selection is bit-identical to the old path on unit-scale, byte-scale and bf16 inputs, 9-11x faster (measured on H200).
• Full stack ran a 17-GPU H3 t2va run resumed from a LoRA checkpoint: log_prob_mean_abs_diff 3.0e-05 (in the verified band), 0 faults; with uint8 on, resp_bytes 1384 -> 436 MiB and train_wait 1010-1140 s -> 486-499 s.
• pre-commit run clean on all touched files; pytest tests/fast: 215 passed.

Checklist

• pre-commit run --all-files passes — run on touched files
• Added/updated tests for new behaviour — fixture updated for the new arg; no new tests
• pytest -x is green — tests/fast, 215 passed
• If launch flags changed, python3 train.py --help still parses
• If a public flag was added, it appears in the CLI reference docs — not updated
• If an example was added, it has a real walkthrough — n/a