Add difficulty_score field to WalkForwardWindow computed via compute_difficulty()
which calculates mean ADX(14) over training bars. Add DifficultyPhase enum (Easy/Mixed/Full)
and filter_by_difficulty() for curriculum-based window filtering — Easy phase trains
only on trending markets (ADX>30), Mixed duplicates trending windows for 2x weight,
Full uses all windows equally. The curriculum_phase field in DQNHyperparameters
(committed in previous ensemble commit) controls the active phase.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
b370b93cc9