Support PDF annotations that span multiple pages

_Updated summary from @robertknight: The Hypothesis client does not allow the user to annotation a selection in a PDF that spans multiple pages._

### Steps

1. Visit https://www.supremecourt.gov/opinions/12pdf/12-96_6k47.pdf
2. This: http://jonudell.net/h/pdf-select-across-pages.mp4

Reproduce the behavior with the linked Supreme Court PDF and the referenced screen recording, confirming that a selection spanning pages cannot be annotated. Trace the PDF selection and annotation flow from that reproduction, then verify that a cross-page selection can be submitted and displayed as one annotation without breaking single-page selections.

Written by the indexing model from the issue text.