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

mkdocs.yml

@@ -7,16 +7,31 @@ repo_name: dtourolle/scene-actor-extraction
 theme:
   name: material
   palette:
-    - scheme: slate
-      primary: indigo
+    - media: "(prefers-color-scheme: dark)"
+      scheme: slate
+      primary: black
+      accent: amber
+      toggle:
+        icon: material/weather-sunny
+        name: Switch to light mode
+    - media: "(prefers-color-scheme: light)"
+      scheme: default
+      primary: black
       accent: indigo
+      toggle:
+        icon: material/weather-night
+        name: Switch to dark mode
   features:
     - navigation.tabs
     - navigation.sections
     - navigation.top
+    - navigation.footer
     - content.code.copy
     - content.code.annotate
 
+extra_css:
+  - stylesheets/extra.css
+
 nav:
   - Home: index.md
   - Findings:
@@ -44,5 +59,8 @@ markdown_extensions:
       base_path: ['.']
       check_paths: true
   - pymdownx.details
+  - pymdownx.emoji:
+      emoji_index: !!python/name:material.extensions.emoji.twemoji
+      emoji_generator: !!python/name:material.extensions.emoji.to_svg
   - attr_list
   - md_in_html