[codex] add deterministic signal overlay replay

[Pigbibi]

Summary

• Add a deterministic overlay replay module for stored shadow signal artifacts
• Add a CLI backtest script plus synthetic price and signal-history examples
• Cover risk-reducing exposure mapping and replay metrics with tests

Validation

• python -m pytest -q
• python scripts/validate_latest_signal.py examples/latest_signal.example.json
• python scripts/backtest_signal_overlay.py --prices examples/price_history.example.csv --signals examples/signal_history --symbol QQQ