The action_select kernel signature changed from single `float epsilon`
to 4 args (eps_start, eps_end, current_epoch, total_epochs) for GPU-side
cosine schedule. The backtest evaluator wasn't updated, causing a
segfault from misaligned kernel args.
Fix: pass eps_start=0, eps_end=0 (greedy in backtest, no exploration).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>