Add ObjectiveMode enum (EpisodeReward/Sharpe) to DQNTrainer and a
TwoPhaseObjective trait + optimize_two_phase() method to ArgminOptimizer.
Phase A optimizes episode reward for fast convergence, Phase B (pending
model Clone support) refines with Sharpe ratio. This keeps the static
extract_objective trait method untouched by separating objective switching
into instance-level state.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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