enhance: embed AppImage update information and publish zsync files

[gengurke]

• Embed gh-releases-zsync update information into the AppImage via appimagetool -u (per AppImageSpec)
• Upload the generated .AppImage.zsync files alongside the AppImages so AppImageUpdate, AppImageLauncher, Gear Lever, AppManager, AM etc. can detect updates and do delta downloads

Fixes #2644

[gengurke]

This PR's workflows are waiting for approval, so I ran the same CI workflow on my fork with this commit. All jobs passed in gengurke/sourcegit run 37514357034.

The Linux package artifacts now include the .zsync files:

Artifact	AppImage	zsync
package.linux-x64	sourcegit-2026.21.linux.amd64.AppImage	sourcegit-2026.21.linux.amd64.AppImage.zsync
package.linux-arm64	sourcegit-2026.21.linux.arm64.AppImage	sourcegit-2026.21.linux.arm64.AppImage.zsync

I downloaded both artifacts and checked them:

• readelf --string-dump=.upd_info shows gh-releases-zsync|sourcegit-scm|sourcegit|latest|sourcegit-*.linux.<arch>.AppImage.zsync, and each file has its own arch in the pattern.
• In each .zsync header, URL is the AppImage file name, and Length and SHA-1 match the AppImage.

I also tested the update flow end to end. I published two temporary pre-releases on my fork, with the update string pointing at the fork. appimageupdatetool found the update and did a zsync delta update, and the result had the same SHA-1 as the new AppImage. Gear Lever's update code (4.6.2) also found the update and downloaded the matching asset.