docs(desktop): land final walkthrough recording

Adds the 3.6 MB take-3 walkthrough mp4 that README already references
via a poster image, and replaces the "coming in a follow-up commit"
placeholder line with a real link (poster now click-through to the
video, caption points at the same file).

Path: examples/desktop/docs/demo-shots/showcase-2026-07-18/
demo-walkthrough.mp4

No source or config changes; the shell tree from a3ca492a is byte-
identical to what it was, this commit just fills in the recording
slot the previous docs commit deferred.

examples/desktop/README.md

@@ -9,9 +9,9 @@ can stay strict.
 
 ![](docs/readme-shots/01-overview.png)
 
-![Demo walkthrough poster](docs/readme-shots/video-poster.png)
+[![Demo walkthrough poster](docs/readme-shots/video-poster.png)](docs/demo-shots/showcase-2026-07-18/demo-walkthrough.mp4)
 
-*Demo walkthrough video: coming in a follow-up commit.*
+*[Demo walkthrough](docs/demo-shots/showcase-2026-07-18/demo-walkthrough.mp4) — 3.6 MB mp4, click poster to play.*
 
 ## Quick start