Dual-mode binary self-detecting via XTRAIN_RANK: launcher spawns one worker per visible GPU forwarding full argv; worker rebuilds config from argv and runs run_worker. CLI flags identical to train_ddp (thread-per-GPU, kept), so it doubles as the before->after throughput driver. thread-per-GPU path untouched. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>