docs: finalize R4 documentation — Dueling 84% Holdout, full ablation record

feat(round3): buffer=80k + target_freq=1500 + shaping=0.5 → 74% holdout, SPL=0.735

feat(round2): extended training, Double DQN 64% holdout, SPL=0.633

feat(round1): baseline DQN variants — Vanilla/Double/Dueling/Double+Dueling