chore: gitignore demo recording files, keep only docs/demo.webp

The demo fixtures, player HTML, and recording scripts are only needed
to regenerate the video. Remove them from tracking and from the
puppeteer devDependency — they stay on disk locally via .gitignore.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: cote-star

.gitignore

@@ -9,3 +9,7 @@ node_modules/
 
 # Editor
 .vscode/
+
+# Demo recording (only docs/demo.webp is committed)
+fixtures/demo/
+scripts/record_demo.*