feat(backtest): wire feature-aware models into backtesting (MLZOO-B.2)

[w7-mgfcode]

Implements PRPs/PRP-MLZOO-B.2-feature-aware-backtesting.md — per-fold leakage-safe X_train/X_future for regression + lightgbm models. Follow-up to PRP-30 DECISIONS LOCKED #6.

[w7-mgfcode]

Implemented in PR #245, merged to dev (merge commit 5a65e35). GitHub did not auto-close because the PR merged into dev, not the default branch.