Migrate sim layer toward TypeScript

[TheSevenPens]

Summary

Project is pure JavaScript. Type safety would help around tablet vs world coordinates and the Three.js / Pen3DSim API surface.

Suggested approach

Incremental: start with Pen3DSim public API + typed tablet/world helpers, then companion files. Keep Svelte UI in JS until sim types settle (or migrate together).

Acceptance

• Core sim API is typed
• Build still works via Vite
• No behavior change required