Add Tarjan SCC + condensation DAG; tidy roadmap

[f-gueguen]

Summary

• Implement Tarjan strongly connected components with stack-based lowlink tracking
• Add condensation DAG builder from SCCs
• Export helpers and types, add docs entries, example and tests
• Clean up ROADMAP formatting for recent items

Testing

• npm run lint
• npm run typecheck
• npm test
• npm run build