Append third CUDA kernel to epsilon_greedy_kernel.cu for Branching DQN:
3 independent epsilon-greedy heads (exposure/5, order/3, urgency/3)
compose to factored action index 0-44 (exposure*9 + order*3 + urgency).
Add Rust composition tests verifying full 45-action coverage and
round-trip decomposition correctness.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>