fix: compress demo GIFs and host in-repo for GitHub rendering

The imgur-hosted GIFs (35-40 MB each) exceeded GitHub's Camo proxy
size limit (~10 MB), causing them to silently fail to render.
Compressed to 4.5 MB and 7.3 MB and moved to media/.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: PABannier

README.md

@@ -2,8 +2,8 @@
 
 State-of-the-art image and video segmentation in portable C/C++
 
-![SAM3 Image Segmentation Demo](https://i.imgur.com/xmnxNP1.gif)
-![SAM3 Video Segmentation Demo](https://i.imgur.com/5y9QcrY.gif)
+![SAM3 Image Segmentation Demo](media/image_demo.gif)
+![SAM3 Video Segmentation Demo](media/video_demo.gif)
 
 ---