A stylus point drawn at 2x zoom maps to the correct normalized page coordinate
(canvas-local 200,300 → scene 100,150 → normalized 0.25,0.25), guarding that ink
stays glued to the page under zoom/pan — the core own-canvas invariant.
flutter analyze lib/editor clean; 238/238 tests (+1).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
