- Atomic checkpoint writes: write to .tmp then fs::rename() (POSIX atomic) - Remove redundant thread::sleep(50ms) after CUDA tensor readback sync - NormStats: bail on missing file instead of computing from test data (lookahead bias) - q_value_std: warn when missing from training metrics instead of silent 0.0 fallback Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>