Ties the engine together: a StrokeHost is anything ink attaches to — a PDF page,
an infinite-board region, or a (P5) CAS overlay — with a stable hostId (cache +
persistence key), a contentSize (normalized↔px mapping), and a revision-tracked
StrokeStore. strokesIn(viewport) broad-phase-culls via stroke_bounds for the
board. The viewport mounts one AnnotationLayer per host; nothing in the engine
knows page vs board (the one host-agnostic ink engine).
Makes StrokeStore (P0) + stroke_bounds load-bearing together. Pure; unit-tested.
flutter analyze lib/editor clean; 208/208 tests (+4).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
