Task 9a: Adaptive atom position training via atom entropy gradient.
Numerical finite-difference gradient on spacing_raw, SGD at LR=1e-3.
Task 9b: Mamba2 BPTT backward through K=8 scan steps.
d_W_A/B/C accumulated, separate Adam at LR=1e-4.
Task 9c: Multi-horizon C51 loss — 3 n-step passes (1/5/20 bar),
3 C51 loss launches, regime-weighted gradient blend (ADX-based).
Fixes the "untrained component" gap identified in plan review.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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