Add Return YouTube Dislike extension example

Add a new example extension that displays YouTube dislike counts using the public Return YouTube Dislike API. This is a simplified port of the Anarios/return-youtube-dislike project adapted for Nebula's native extension runtime (content-script only, no Chrome APIs). Includes proper attribution and respects rate limits and GPL-3.0 licensing requirements.

examples/extensions/return-youtube-dislike/README.txt

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+Return YouTube Dislike (Nebula)
+===============================
+
+Shows estimated YouTube dislike counts using the public
+Return YouTube Dislike API.
+
+Based on / inspired by:
+  https://github.com/Anarios/return-youtube-dislike
+  https://returnyoutubedislike.com
+
+API attribution (required by RYD):
+  https://returnyoutubedislike.com
+
+License:
+  This Nebula package is a simplified derivative intended for use with
+  Nebula's native extension runtime. Upstream project is GPL-3.0.
+  See LICENSE.txt.
+
+Install
+-------
+Copy this folder to:
+
+  %LOCALAPPDATA%\Nebula\User Data\extensions\return-youtube-dislike\
+
+Open Settings > Plugins > Reload Plugins, then open a YouTube video.
+
+Notes
+-----
+- Content-script only (no Chrome extension APIs)
+- No dislike vote submission to RYD
+- No premium analytics UI
+- Respect RYD rate limits (100/min, 10k/day)