Each canvas has two coordinate systems: media (CSS px, what you reason
about) and bitmap (device px = media × devicePixelRatio, the backing
buffer). Drawing 1px lines in the bitmap scope with integer snapping keeps
them crisp on retina/HiDPI displays.
HiDPI canvas handling (ARCHITECTURE.md §3.1).
Each canvas has two coordinate systems: media (CSS px, what you reason about) and bitmap (device px = media × devicePixelRatio, the backing buffer). Drawing 1px lines in the bitmap scope with integer snapping keeps them crisp on retina/HiDPI displays.