feat: migrate to Bun runtime for 2.8-4.4x performance boost

- Complete Node.js → Bun migration with zero breaking changes
- Replace @types/node + ts-node with bun-types
- Update build scripts to use bun instead of node/npm
- Add bun test command and update engine requirement
- All existing npx/pnpx commands continue working
- Update docs to recommend bunx for optimal performance

Author: delorenj