GPU: Add sparse solvers, FFT, and SpMV (cuDSS, cuFFT, cuSPARSE)

Add GPU sparse direct solvers (Cholesky, LDL^T, LU) via cuDSS, 1D/2D FFT
via cuFFT with plan caching, and sparse matrix-vector/matrix multiply
(SpMV/SpMM) via cuSPARSE.

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

Eigen/src/GPU/CuSparseSupport.h

@@ -0,0 +1,34 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2026 Rasmus Munk Larsen <rmlarsen@gmail.com>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// cuSPARSE support utilities: error checking macro.
+
+#ifndef EIGEN_GPU_CUSPARSE_SUPPORT_H
+#define EIGEN_GPU_CUSPARSE_SUPPORT_H
+
+// IWYU pragma: private
+#include "./InternalHeaderCheck.h"
+
+#include "./GpuSupport.h"
+#include <cusparse.h>
+
+namespace Eigen {
+namespace internal {
+
+#define EIGEN_CUSPARSE_CHECK(x)                                                 \
+  do {                                                                          \
+    cusparseStatus_t _s = (x);                                                  \
+    eigen_assert(_s == CUSPARSE_STATUS_SUCCESS && "cuSPARSE call failed: " #x); \
+    EIGEN_UNUSED_VARIABLE(_s);                                                  \
+  } while (0)
+
+}  // namespace internal
+}  // namespace Eigen
+
+#endif  // EIGEN_GPU_CUSPARSE_SUPPORT_H