Move engine (scene-graph, renderer, layout, codec, kiwi) into
packages/core/ as a Bun workspace package. The app's src/engine/
files become thin re-export shims so all existing imports still work.
- Bun workspace with packages/core
- Zero DOM dependencies in core
- Headless-friendly CanvasKit init (locateFile option)
- Constants split: engine constants in core, UI constants in app
- All 76 unit tests pass, lint + typecheck clean, vite build works
6d1cec1606