Cross-module finding correlation engine.

Runs after all scanner stages complete. Applies a set of [CorrelationRule]s that look for patterns spanning multiple scanners — e.g., SSRF + Redis, or source-code exposure + hardcoded secrets — and emits new chain findings.

Adding a new rule

1. Create src/rules/{rule}.rs and implement [CorrelationRule].
2. Register it in [CorrelationEngine::default()]. That's the only change needed.