Adds hitTestCanvasNode (topmost node under a point, cards over the group
frames behind them, via the same viewport the painter draws with) and the
CanvasView widget: scroll-zoom, drag-pan, and click-to-select with a focus
ring. Node drag/resize and edit affordances follow.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>