Add Python bindings for CUDA cluster finder

- Add bind_ClusterFinderCUDA.hpp with pybind11 bindings for
  ClusterFinderCUDA
- Build CUDA bindings as separate _aare_cuda.so to avoid
  segfaults from mixing nvcc and gcc compiled code in the
  same shared object
- Re-export CUDA classes onto _aare in __init__.py so user
  code uses `from aare import ClusterFinderCUDA` regardless
  of which .so hosts the class
- Factory in ClusterFinder.py selects backend; RuntimeError
  if GPU requested on CPU-only build
- Update python/CMakeLists.txt: _aare_cuda module gated
  behind AARE_CUDA and AARE_PYTHON_BINDINGS
- Add validation notebook: ~20x speedup vs sequential ClusterFinder

python/src/cuda_bindings.cu

@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: MPL-2.0
+//
+// CUDA-only Python extension module. Registers ClusterFinderCUDA along with
+// the ClusterVector and Cluster types it exposes in its return values, so
+// the module is self-contained — users can call steal_clusters() and get
+// back a usable ClusterVector without _aare needing to be imported first.
+
+#include "bind_Cluster.hpp"
+#include "bind_ClusterVector.hpp"
+#include "bind_ClusterFinderCUDA.hpp"
+
+#include <pybind11/pybind11.h>
+
+namespace py = pybind11;
+
+// Register the Cluster + ClusterVector pair for one (T, N, M) combination.
+// Subset of DEFINE_CLUSTER_BINDINGS from module.cpp: we register what
+// ClusterFinderCUDA actually returns, nothing more. File I/O, eta and
+// reduce_to_2x2 stay on the CPU side.
+#define DEFINE_CUDA_CLUSTER_TYPES(T, N, M, U, TYPE_CODE)                       \
+    define_ClusterVector<T, N, M, U>(m, "Cluster" #N "x" #M #TYPE_CODE);       \
+    define_Cluster<T, N, M, U>(m, #N "x" #M #TYPE_CODE);
+
+#define DEFINE_BINDINGS_CLUSTERFINDER_CUDA(T, N, M, U, TYPE_CODE)              \
+    aare::define_ClusterFinderCUDA<T, N, M, U>(                                \
+        m, "Cluster" #N "x" #M #TYPE_CODE);
+
+PYBIND11_MODULE(_aare_cuda, m) {
+
+    // Types first — finders reference them in their signatures.
+    // SFINAE excludes 2x2 on ClusterFinderCUDA, so we skip it here too.
+    DEFINE_CUDA_CLUSTER_TYPES(int,    3, 3, uint16_t, i);
+    DEFINE_CUDA_CLUSTER_TYPES(double, 3, 3, uint16_t, d);
+    DEFINE_CUDA_CLUSTER_TYPES(float,  3, 3, uint16_t, f);
+
+    DEFINE_CUDA_CLUSTER_TYPES(int,    5, 5, uint16_t, i);
+    DEFINE_CUDA_CLUSTER_TYPES(double, 5, 5, uint16_t, d);
+    DEFINE_CUDA_CLUSTER_TYPES(float,  5, 5, uint16_t, f);
+
+    DEFINE_CUDA_CLUSTER_TYPES(int,    7, 7, uint16_t, i);
+    DEFINE_CUDA_CLUSTER_TYPES(double, 7, 7, uint16_t, d);
+    DEFINE_CUDA_CLUSTER_TYPES(float,  7, 7, uint16_t, f);
+
+    DEFINE_CUDA_CLUSTER_TYPES(int,    9, 9, uint16_t, i);
+    DEFINE_CUDA_CLUSTER_TYPES(double, 9, 9, uint16_t, d);
+    DEFINE_CUDA_CLUSTER_TYPES(float,  9, 9, uint16_t, f);
+
+    // Finders
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(int,    3, 3, uint16_t, i);
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(double, 3, 3, uint16_t, d);
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(float,  3, 3, uint16_t, f);
+
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(int,    5, 5, uint16_t, i);
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(double, 5, 5, uint16_t, d);
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(float,  5, 5, uint16_t, f);
+
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(int,    7, 7, uint16_t, i);
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(double, 7, 7, uint16_t, d);
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(float,  7, 7, uint16_t, f);
+
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(int,    9, 9, uint16_t, i);
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(double, 9, 9, uint16_t, d);
+    DEFINE_BINDINGS_CLUSTERFINDER_CUDA(float,  9, 9, uint16_t, f);
+}
+
+#undef DEFINE_CUDA_CLUSTER_TYPES
+#undef DEFINE_BINDINGS_CLUSTERFINDER_CUDA