When AGENTIC_STEP_LOG_PATH is set, the scheduler emits one JSONL line
per scheduler step with t_unix, worker_id, prefill/decode token
counts, n_running/n_waiting, preempted ids, and per-request phase
labels. No-op when the env var is unset, so production engines are
not impacted. bench.sh now threads AGENTIC_STEP_LOG_DIR through to
each per-engine launch so step logs end up at engine_${i}.jsonl.
Required by Batch 2 (PD-colo interference index) and Batch 5
(same-worker overlap attribution); engine /metrics polling cannot
provide per-step granularity.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
