Remove per-epoch GPU data freeing that forced re-upload every epoch.
Training data within a walk-forward window is immutable — uploading once
and keeping it GPU-resident eliminates the init phase entirely.
Epoch time: 153ms → 78ms on RTX 3050 (epochs 2+).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
