Refocus on USD/JPY and add walk-forward backtester with regime filter

- Strip multi-pair plumbing; drop synthetic-data fallback
- Add BacktestConfig + walk-forward replay through the live signal pipeline
- Add metrics module (PF, Sharpe, Sortino, max DD, trade journal)
- Add parallel grid optimizer with disk parquet cache
- Add H4 ADX regime gate (--regime-filter, --regime-min-adx) — cuts chop
  losses ~84% in 2023 H1 while preserving trend edge

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

requirements.txt

@@ -1,3 +1,5 @@
 numpy>=1.24
 pandas>=2.0
 requests>=2.28
+pyarrow>=14.0
+tqdm>=4.66