Delete stub rainbow_agent.rs (fake select_action, hardcoded train loss)
and promote rainbow_agent_impl.rs to rainbow_agent.rs. Unify on the
real 8-field RainbowAgentMetrics from rainbow_config.rs, replacing the
4-field stub version (epsilon→exploration_rate, average_loss→current_loss).
5 files changed: -667/+447 lines, 2698+32 tests pass, 0 clippy.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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