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

docs/stylesheets/extra.css

@@ -0,0 +1,24 @@
+/* Frames and charts presented as cards */
+.md-typeset img {
+  border-radius: 6px;
+}
+.md-typeset p > img:only-child {
+  box-shadow: 0 2px 12px rgba(0, 0, 0, 0.35);
+}
+
+/* Captions: an italic-only paragraph immediately after an image reads as a
+   figure caption — centered, small, muted. Falls back to plain italics in
+   browsers without :has(). */
+.md-typeset p:has(> img:only-child) + p > em:only-child {
+  display: block;
+  text-align: center;
+  font-size: 0.72rem;
+  line-height: 1.5;
+  color: var(--md-default-fg-color--light);
+  margin-top: -0.4rem;
+}
+
+/* Slightly tighter hero image spacing on the landing page */
+.md-typeset h1 + p + p > img:only-child {
+  margin-top: 0.2rem;
+}