feat: add top-down movement controller

[f-gueguen]

Summary

• add a top-down movement controller with configurable acceleration, deceleration, drag, and diagonal normalization
• expose new gameplay exports, documentation updates, roadmap progress, and example entry
• add targeted unit tests covering acceleration, deceleration, diagonal movement, and resets

Testing

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