Add GPU overrides for _ind_union in CUDA and AMDGPU extensions

Mirrors the existing `_ind_intersect` overrides: collect GPU index vectors
to CPU before computing the union, since `union` does not work on GPU arrays.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: lkdvos

ext/MatrixAlgebraKitAMDGPUExt/MatrixAlgebraKitAMDGPUExt.jl

@@ -161,9 +161,11 @@ function MatrixAlgebraKit._mul_herm!(C::StridedROCMatrix{T}, A::StridedROCMatrix
     return C
 end
 
-# TODO: intersect doesn't work on GPU
+# TODO: intersect/union don't work on GPU
 MatrixAlgebraKit._ind_intersect(A::ROCVector{Int}, B::ROCVector{Int}) =
     MatrixAlgebraKit._ind_intersect(collect(A), collect(B))
+MatrixAlgebraKit._ind_union(A::ROCVector{Int}, B::ROCVector{Int}) =
+    MatrixAlgebraKit._ind_union(collect(A), collect(B))
 
 function _sylvester(A::AnyROCMatrix, B::AnyROCMatrix, C::AnyROCMatrix)
     hX = sylvester(collect(A), collect(B), collect(C))

ext/MatrixAlgebraKitCUDAExt/MatrixAlgebraKitCUDAExt.jl

@@ -165,9 +165,11 @@ function MatrixAlgebraKit._mul_herm!(C::StridedCuMatrix{T}, A::StridedCuMatrix{T
     return C
 end
 
-# TODO: intersect doesn't work on GPU
+# TODO: intersect/union don't work on GPU
 MatrixAlgebraKit._ind_intersect(A::CuVector{Int}, B::CuVector{Int}) =
     MatrixAlgebraKit._ind_intersect(collect(A), collect(B))
+MatrixAlgebraKit._ind_union(A::CuVector{Int}, B::CuVector{Int}) =
+    MatrixAlgebraKit._ind_union(collect(A), collect(B))
 
 function _sylvester(A::AnyCuMatrix, B::AnyCuMatrix, C::AnyCuMatrix)
     # https://github.com/JuliaGPU/CUDA.jl/issues/3021