Add Mosaic Mod Manager - #3868

Thanks for the automated review — wanted to leave context on why it failed, in case a maintainer looks at this.

This AppImage is built with quick-sharun / pkgforge-dev/appimagetool, which uses DwarFS (not squashfs) as the embedded filesystem, embedding the uruntime AppImage runtime. That's a deliberate choice for this build — DwarFS's block-level deduplication compresses the many similar .so files an AppImage like this bundles noticeably better than squashfs does.

The AppImage's own bundled runtime handles DwarFS natively and works correctly for end users (confirmed working directly and via Gear Lever and topgrade), but this repo's test harness deliberately mounts submissions with the classic reference runtime-fuse2-x86_64 runtime rather than the AppImage's own bundled one, and that runtime only understands squashfs — hence This doesn't look like a squashfs image. It's a real format difference, not a broken build.

Happy to answer any questions. Leaving this PR open in case a squashfs-based build is a hard requirement for listing, or in case DwarFS-based AppImages are already handled some other way here — no action needed on my end otherwise.