feat: add flow field pathfinding

[f-gueguen]

Summary

• implement uniform-cost flow field builder returning integration costs and normalized vectors
• document API across docs, README, and project guide; track progress in roadmap
• add runnable example and Vitest coverage ensuring cost gradient and zero flow for blocked tiles

Testing

• npm run lint
• npm run typecheck
• npm run build
• npm run size
• npm run test:coverage