Eliminates per-frame WASM allocations for shadows and blurs:
- Reusable effectLayerPaint instead of new Paint() per effect
- ImageFilter cache keyed by params (drop shadow, blur, decal blur)
- Zero allocations during SkPicture recording for cached effects
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
