Fix 1 (HIGH): Ensemble trainers were created + data uploaded inside the
fold loop, causing redundant GPU uploads per fold per ensemble member.
Moved creation + init_from_fxcache before the fold loop; inside the loop
we now reuse trainers via set_training_range + reset_for_fold.
Fix 2 (MEDIUM): reset_for_fold cleared gpu_data = None, forcing re-upload
on every fold even though the full dataset was already GPU-resident via
init_from_fxcache. Removed the clearing — data stays on GPU across folds.
Fix 3 (LOW): train_fold_from_slices allocated a Vec<f64> per bar via
t.to_vec(). Added train_with_data_full_loop_slices that accepts
&[([f64; 42], [f64; 4])] — both are Copy stack types, zero heap alloc
per element. Added matching collect_gpu_experiences_slices and
run_training_steps_slices helpers.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>