Remove: ReplayBuffer, Transition, NStepEntry, push_to_replay, sample_and_gather, n_step_buffer, replay.rs, --diag-every flag. PER call sites stubbed with todo!() — replaced by GPU PER in next commits. Also fixes pre-commit hook to allow todo!() macro (per CLAUDE.md: "todo!() macro is OK for runtime stubs") while still blocking TODO/FIXME comment markers. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>