perf(reassembler): batch group staging reads and expose fetch timings

[pthombre]

What does this PR do?

Token-capture reassembly currently fetches staging rows once per rollout and combines data-plane latency with verification time. Fetch the deduplicated staging-key union once per group, preserve per-rollout failure isolation, and expose separate fetch and verification timings.

• Healthy groups use one base read and at most one targeted route-payload read in direct mode. Failed batches subdivide to isolate missing, malformed, or unready rows.
• Retain existing aggregate metrics and public Gym/source interfaces.
• Prefer the creating node for reassembler actors, with soft affinity and explicit spill when that node is busy.

Issues

Closes #4010. Follow-up to #3837.

PR stack

This is the first PR in the stack: main → pranav/reassembler-group-fetch → pranav/capture-orchestration.

Next in stack: #4073 (capture orchestration).

The capture-orchestration PR builds on this branch. Merge this PR first, then rebase/retarget the dependent PR to main.

Validation

• Slurm job 3656922, final commit 364119fa60553475f3b8022242c7f40a873d032b: completed with exit 0:0.
• 44 live TQ/Gym tests + 20 actor tests passed, zero failures or skips; includes 18 new regression cases.
• Scoped pyrefly: zero errors; Ruff, import sorting, formatting, and diff checks passed.
• Used the existing rl-gym container and its Gym environment. Live storage coverage uses TQ's simple backend; full training throughput and multi-node placement were not benchmarked.

Before ready for review

• Followed contributor guidelines; commits include sign-off trailers.
• Added and ran relevant unit/regression tests through Slurm.
• Updated affected API docstrings and metric semantics.
• Run a representative full training/functional workload.

This PR is intentionally a draft.

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