CMake: Preserve CUDA flag initialization during compiler detection

Seed detection flags from CUDAFLAGS and CMAKE_CUDA_FLAGS_INIT only when CMAKE_CUDA_FLAGS is undefined, then use CMake's initialized cache for build flags. Keep Eigen additions outside the cache so reconfiguration preserves user defaults without retaining old Eigen flags.

Add a build-system regression using CMake's CUDA flag initializer, covering environment and toolchain defaults, late compiler defaults, explicit and empty overrides, Clang routing, and reconfiguration.