chore(deps): upgrade rollup 4, vitest 2, typescript 6, eslint 9

Major version bumps for core build and dev tooling:
- rollup 2 → 4 (with matching plugin upgrades)
- vitest 0.28 → 2.1.9 (Node 18 compatible)
- typescript 4.9 → 6.0 (moduleResolution: bundler)
- eslint 8 → 9 (flat config migration, typescript-eslint)
- Remove @babel/eslint-parser and @babel/plugin-proposal-object-rest-spread

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: krisk