Move mixed_precision, xavier_init, portfolio_tracker, action_space,
order_router from dqn/ to ml/src/ root. Extract TradingAction enum
from dqn/agent.rs to standalone trading_action module. Re-export
from dqn/mod.rs for backward compatibility.
These modules are shared infrastructure used by PPO, trainers,
hyperopt, and ensemble — not DQN-specific.
Test results: 2757 passed, 0 failed, 25 ignored (unchanged).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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