Notes now use the single performant inking engine (PenCanvas) instead of
the old ink_canvas, per "all note features on the pen-first canvas".
- ink_stroke_adapter: pure InkStroke<->PenStroke bridge (normalize against
a logical note page; drop non-freehand shapes/text). Round-trip tested.
- pen_palette_widgets: shared M3 ToolButton/PaletteDivider/RoundIconButton
so PDF + note editors use identical chrome (PenEditorScreen migrated to
them; its private copies deleted).
- PenNoteScreen: PenCanvas over a white logical page, undo/redo, title,
save -> Note.strokes (+ local OCR for search). Pressure curve, eraser
size/mode and palm rejection all inherited from the shared canvas.
- Route home (new/open) + search note hits -> PenNoteScreen; remove the
now-redundant "Pen Canvas (beta)" spike button; delete the dead old
note_editor_screen.
Tests: ink_stroke_adapter (5) + pen_note_screen widget (load + commit, 2).
flutter analyze: 0 issues. Full suite: 265/265.
