fix: suppress unsafe_code warnings on GPU regime kernel launches

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

crates/ml/src/cuda_pipeline/gpu_walk_forward.rs

@@ -655,6 +655,7 @@ impl GpuWalkForwardData {
 
 /// Classify regimes on GPU and compute prefix sums for O(1) range queries.
 /// Returns prefix sums: `prefix[r * (n+1) + i]` = count of regime `r` in `[0, i)`.
+#[allow(unsafe_code)]
 pub fn classify_regimes_gpu(
     stream: &Arc<CudaStream>,
     features_f32: &CudaSlice<f32>,