[codex] Offload orchestrator rollout postprocess

[samsja]

Summary

Moves the CPU-heavy rollout postprocess path out of the orchestrator event loop behind one worker boundary.

• Adds build_training_batch_and_metrics(...) in prime_rl.orchestrator.postprocess for advantages, filters, accepted-rollout save, image offload, token backfill, rollout interleaving, training batch construction, and rollout metric aggregation.
• Leaves the main coroutine responsible for async-only work: scheduler retry control, teacher logprobs, trainer send, monitor logging, heartbeat, and cleanup.
• Preserves accepted-rollout artifact behavior by saving train_rollouts.jsonl before image offload, and preserves all-filtered retry behavior without writing rollout artifacts for rejected attempts.

Validation

• uv run ruff check src/prime_rl/orchestrator/orchestrator.py src/prime_rl/orchestrator/postprocess.py
• uv run pytest tests/unit/orchestrator/test_advantage.py tests/unit/orchestrator/test_filters.py tests/unit/orchestrator/test_trajectories.py tests/unit/orchestrator/test_sft_trajectories.py tests/unit/orchestrator/test_orchestrator_setup.py -q