Adds crates/ml/tests/gpu_backtest_validation.rs with 6 deterministic
tests that validate GpuBacktestEvaluator produces reasonable metrics:
always-long on uptrend (positive PnL), always-long on downtrend
(negative PnL), always-flat (~zero PnL), multi-window ordering,
extended metrics finiteness/self-consistency, and trade count.
All tests are #[ignore] gated and skip gracefully on CPU-only machines
(CI passes with 6 ignored; intended for GPU development machines).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
f19cd4b26a