Adds RL_N_STEP_INDEX (slot 542, default 10), n_step_gamma field to Transition, and NStepEntry ring buffer struct to IntegratedTrainer. Remaining: implement n-step accumulation in push_to_replay and modify bellman_target_projection.cu to use n_step_gamma. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>