Exports selected strategy pages as an mp4 slideshow: each page held for a
configurable step duration (1–30s, persisted), with full-fidelity animated
page transitions between consecutive included pages (420ms @ 60fps, same
easing and entry diffing as the in-app page switch).

- Extract transition entry diffing (TransitionPlanner) and t-parameterized
  rendering (TransitionEntriesLayer) so the live overlay and exporter share
  one implementation
- Drawings and images now fade in early during page transitions (in-app and
  in exported video)
- Encode via bundled LGPL ffmpeg.exe (h264_mf, mpeg4 fallback) spawned as a
  child process; scripts/fetch_ffmpeg.ps1 stages a pinned, checksummed BtbN
  build during release packaging
- Export Video toolbar button + settings dialog (page checkboxes, duration
  slider, progress/cancel)
- Design docs: docs/adr/0001–0004, glossary in CONTEXT.md