Adds a standalone CUDA kernel that converts DQN exposure indices (0-4)
to factored action indices (0-44) entirely on GPU, reusing the existing
route_order() device function from common_device_functions.cuh. Wired
into GpuActionSelector as route_exposure_to_factored() method, following
the same DtoD copy pattern as the existing select_actions methods. This
eliminates a GPU->CPU->GPU roundtrip when post-hoc routing is needed
after epsilon_greedy_select.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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