Adds a sliding-window walk-forward harness for the T10 backtest:
- New load_snapshots_from_fxcache_at(start_offset, ...) loader variant
reads bars [start_offset..start_offset+max_snapshots) from the fxcache.
Alpha-cache lookups use absolute bar indices, so the same
alpha_logits_cache.bin works across folds.
- New --data-start-offset CLI flag on alpha_compose_backtest.
- scripts/walk_forward_cv.sh runs 3 folds (window=700K, train_frac=0.6)
at offsets 0 / 600K / 1.2M, producing /tmp/cv_fold_{A,B,C}.json plus
an aggregated mean±stddev Sharpe table across folds.
Walk-forward result (alpha_logits_cache trained on bars 0..1.57M, so
fold C eval is fully past the stacker cut):
cost fold-A fold-B fold-C mean ± stddev
0.0000 +91.52 -21.44 +46.74 +38.94 ± 56.88
0.0625 +84.94 -27.97 +38.42 +31.79 ± 56.74
0.1250 +79.91 -31.22 +33.51 +27.40 ± 55.82
0.2500 +72.77 -45.41 +15.16 +14.17 ± 59.09
0.5000 +50.52 -59.82 -12.75 -7.35 ± 55.37
Fold B (mid-quarter, bars 600K..1.3M) is a disaster — win rate
collapses to 0-22% across all costs. Folds A and C succeed strongly.
Cross-fold SD ≈ mean, so the policy is regime-dependent and cannot
be reliably deployed without regime detection.
Mean Sharpe at half-tick (+27.40) is still ~7× the stateless
Phase 1d.4 baseline (-4.0), so the temporal encoder adds real value
on average — but the single-window +62 OOS celebrated earlier was
a cherry-picked favorable regime, not a deployment-ready result.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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