docs: montage-renderer imagery, visual polish, README screenshots

- switch report frames to the scene best/worst montage renderer
  (Onscreen/Offscreen panels + TPI/FPI/FN legend): perfect-second hero,
  wedding couple, funeral 19-of-20, polygraph bridging, crew-scene FN
  ceiling, Robert Patrick ground-truth gap, rapid-cut double label,
  Herbie Hancock on an in-fiction screen
- deep dive restructured: extinction bridging framed as designed
  behavior with a measurable cost (debug overlay draws the boxes; the
  shipped output is presence windows), plus the face-vs-presence
  ceiling and two X-Ray-is-wrong exhibits
- Material polish: light/dark palette toggle, landing-page grid cards,
  figure/caption CSS, how-to-read admonition; site_url set so 404 links
  resolve under the Pages subpath
- README: perfect-second and screen-call frames committed (gitignore
  exceptions), readme_example.jpg retired
- build_site.sh: stage_frame helper downscales montage frames to 1920px
  and pulls any missing montage-frames packages

README.md

@@ -8,9 +8,18 @@ discussion in the [deep dive](https://pages.tourolle.paris/dtourolle/scene-actor
 Full benchmark write-up, model comparison, and failure-mode analysis:
 **https://pages.tourolle.paris/dtourolle/scene-actor-extraction/**
 
-![Example: correctly identified actors in a held-out film](docs/assets/images/readme_example.jpg)
-*A held-out film (never used for threshold tuning) — three actors correctly
-identified with calibrated confidence scores.*
+![A perfect X-Ray second on a held-out film](docs/assets/images/lovelace_perfect_second.jpg)
+*A perfect X-Ray second on a held-out film (never used for threshold tuning):
+every visible face named at 100%, the background extra honestly left unnamed,
+and the two credited cast without a visible face correctly carried as present
+off-screen. Bottom panels show the per-second verdict against Amazon X-Ray
+(green = correct, orange = wrong, blue = missed).*
+
+It also doesn't care whether the face is in the room:
+
+![Herbie Hancock identified on an in-fiction video-call screen](docs/assets/images/valerian_screen_call.jpg)
+*Herbie Hancock at 98% — as a face on a screen inside the movie, under a
+sci-fi HUD overlay.*
 
 ## How it works