Replace the single-page PdfPageView bitmap with a pdfrx
PdfViewer: real vector text, continuous scroll, native
pinch-zoom (no custom zoom solver, so no zoom-jump here).
Ink is glued per-page via pageOverlaysBuilder; the pen is
captured at the viewer level by PenCaptureRegion while touch
falls through to scroll / pinch / text-select.
Add select-text -> highlight via PdfTextSelectionParams: the
selection's fragment rects are stored as normalized page rects
and drawn under the ink. In-memory only for now.
Per-page persistence, undo/redo, tools, colors, thumbnails and
pen settings are reused verbatim. analyze clean, 270 tests green.