Poster frames for the walkthrough videos

The player was showing a blank white rectangle, which reads as broken. That
is frame zero of each recording — the instant before the page painted. Only
frame zero is white (by 0.5s the real UI is up), but frame zero is exactly
what a browser picks when no poster is set.

Each video now has a poster pulled from ~30% in, where the tool is in a
meaningful state rather than mid-intro. Posters live beside the videos in the
volume and the player derives the URL from the video's own, so adding a new
walkthrough needs no page change.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

public/tool-video.js

@@ -42,6 +42,10 @@
 
     var vid = document.createElement('video');
     vid.controls = true;
+    // Poster lives beside the video (same slug, .jpg). Without it the browser
+    // shows frame zero, and frame zero is the instant before the page painted
+    // — a blank white rectangle that reads as a broken player.
+    vid.poster = SRC.replace(/\.mp4$/, '.jpg');
     // Nothing is fetched until the panel is actually opened; open() then bumps
     // this to 'metadata' so an expanded panel shows a real first frame rather
     // than a black rectangle.