Closes recs 2 + 3 in lookahead-bias-audit-2026-04-28.md.
Fix 2A (rec 2): add walk_forward::PURGE_BARS = 5 constant and insert
val_start = train_end + PURGE_BARS in all 4 fold-construction sites:
generate_walk_forward_indices, _from_timestamps, _windows (date-based,
drops first PURGE_BARS bars of val), and gpu_walk_forward::generate_folds
(both stratified variants preserve the gap on boundary shift). 5-bar
width matches max per-bar feature lookback (autocorr lags 1/5/10);
compile-time per feedback_isv_for_adaptive_bounds since the
feature-pipeline lookback is itself compile-time.
Fix 2B (rec 3): per-fold NormStats fit from train-only data.
- walk_forward.rs: add from_features_slice + denormalize/denormalize_batch
helpers (inverse of normalize_batch for clamp-bounded round-trip).
- train_baseline_rl.rs fold loop: load fxcache sidecar norm_stats.json,
denormalise back to RAW, refit per-fold via from_features_slice,
renormalise full dataset, re-upload via init_from_fxcache, save
per-fold norm_stats_fold{N}.json. DBN-fallback path keeps legacy
behaviour (no sidecar to denormalise from) with a warn! log.
Ensemble trainer block is documented follow-up (separate code path).
Behavioral tests:
- test_norm_stats_per_fold_fit_train_only: synthetic train-mean=1.0 /
val-mean=9.0 dataset; from_features_slice recovers train-only mean to
ε=1e-5 while from_features returns global 5.0
- test_norm_stats_denormalize_roundtrip: non-clamped features round-trip
bit-identically
- test_walk_forward_purge_gap_indices_from_timestamps: every fold has
val_start - train_end == PURGE_BARS
- test_fold_generation_basic (gpu_walk_forward): updated to expect the
purge gap
Validation: cargo check --workspace clean (12.30s); 20/20 walk_forward
+ gpu_walk_forward tests pass; audit_sp18_consumers.sh --check exit 0.
Pre-existing test failures on local RTX 3050 Ti are unrelated SIGSEGVs
(VRAM exhaustion in chunked Thompson tests, pre-existing on baseline).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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