fix: migrate 44 test files from Candle to native CUDA — zero test compile errors
Complete Candle→cudarc migration for all test code. The workspace
now compiles clean with `cargo check --workspace --tests` (0 errors)
and `cargo clippy --workspace --lib -D warnings` (0 errors).
Migration patterns applied across all files:
- Tensor → GpuTensor (from_host, zeros, randn, full)
- Device → MlDevice (cuda, cuda_if_available, new_cuda)
- All GpuTensor ops now take &Arc<CudaStream>
- VarMap/VarBuilder → GpuVarStore or removed
- DType removed (everything f32)
- Candle autograd tests (Var, GradStore, backward) → #[ignore]
- Preprocessing tests → host-side Vec<f32> (CPU-side by design)
- PPO hidden state → host-side Vec<f32> slices
- UnifiedTrainable: forward_loss(&[f32], &[f32]) → f64
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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