pdfcraft_render::transition gives the picture of a page transition at a
moment of its run: which parts of the outgoing and incoming page to paint,
where, and how opaque, in page fractions, for all twelve styles and every
direction. Pure and tested; the full-screen presentation will paint from
it when it plays transitions.
Refs #379.
Claude-Session: https://claude.ai/code/session_01RUNQSPVXx3dDBabRZHAnjz
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>
