strokeBounds (tight AABB over normalized points, null for empty, zero-size for a
single point), strokesBounds (union), and strokeIntersects (does a stroke's box
overlap a viewport rect — touching edges count). Broad-phase primitive for the
infinite board: skip painting/erasing/hit-testing strokes off-screen (R1 perf),
and a cheap pre-filter before the exact per-point eraser test.
Pure geometry over EditorStroke; fully unit-tested.
flutter analyze lib/editor clean; 191/191 tests (+10).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
