PPT slides now annotate with the single performant inking engine
(PenCanvas) instead of the old ink_canvas, per "all note features on the
pen-first canvas".
- PenSlideScreen: per-slide normalized strokes over each slide image,
prev/next + slider nav, undo/redo, shared M3 palette, and the pressure
curve / eraser size+mode / palm rejection from the shared canvas.
- slide_export: pure, tested export geometry. Because strokes are now
normalized to the page rect, the PDF exporter maps them straight into
each slide's draw rect — fixing the old exporter's known ink
misalignment (it guessed live-widget size).
- Route PPT import + open -> PenSlideScreen; delete the dead old
ppt_annotator_screen. (ink_canvas/annotation_toolbar remain for
split_view, the last old-canvas screen.)
Tests: slide_export geometry (4). flutter analyze: 0 issues. Suite: 269/269.
