fix: backtest action_select missing isv_signals_ptr + regenerated fxcache

Backtest evaluator missing isv_signals_ptr arg (passed as NULL).
Regenerated test fxcache with OFI_DIM=20 from MBP-10 + trades data.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/ml/src/cuda_pipeline/gpu_backtest_evaluator.rs

@@ -981,6 +981,7 @@ impl GpuBacktestEvaluator {
                     .arg(&1.0_f32)     // eps_urg_mult (no-op at eps=0)
                     .arg(&(chunk_start as i32)) // timestep for stateless Philox RNG
                     .arg(&0u64)                // per_sample_epsilon: NULL → use cosine schedule
+                    .arg(&0u64)                // isv_signals_ptr: NULL → static hold (backtest)
                     .launch(launch_cfg)
                     .map_err(|e| MLError::ModelError(format!(
                         "chunked action_select chunk_start={chunk_start}: {e}"