Linux: AppImage update information and a .zsync beside it (#349) (#399)

* Linux: AppImage update information and a .zsync beside it (#349)

appimagetool now embeds
gh-releases-zsync|<owner>|<repo>|latest|photocraft-*-linux-<arch>.AppImage.zsync
(owner/repo from GITHUB_REPOSITORY, default storytold/photocraft), so
AppImageUpdate, AppImageLauncher and the like can find the next release and
fetch only the blocks that changed. With zsyncmake on the host it also writes
the .zsync, into its working directory, so the script runs it in $DIST and
warns when the file is missing.

The release job installs zsync and checks the embedded string and the .zsync
before uploading; docs/releasing.md lists the new asset.

Verified locally with packaging/linux/package.sh --skip-build --formats
appimage and zsyncmake from Ubuntu 22.04: --appimage-updateinformation prints
the string, and the .zsync's URL is the AppImage's basename.

* package.sh: absolute AppDir and appimagetool paths before running in $DIST

appimagetool now runs inside $DIST so zsyncmake writes the .zsync there,
which broke relative CARGO_TARGET_DIR / APPIMAGETOOL paths (AGENTS.md
suggests target/agent-<name>). Resolve both first, remove a stale .zsync
so a missing zsyncmake is reported, and list the .zsync and zsyncmake in
the header.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: bflatastic <bombayapirat@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -210,7 +210,7 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y --no-install-recommends \
             libxkbcommon-dev libwayland-dev libx11-dev libxrandr-dev libxi-dev libgl1-mesa-dev \
-            desktop-file-utils appstream python3-yaml
+            desktop-file-utils appstream python3-yaml zsync
           curl -fsSL -o "$RUNNER_TEMP/nfpm.deb" \
             "https://github.com/goreleaser/nfpm/releases/download/v${NFPM_VERSION}/nfpm_${NFPM_VERSION}_${{ matrix.deb }}.deb"
           sudo dpkg -i "$RUNNER_TEMP/nfpm.deb"
@@ -235,6 +235,9 @@ jobs:
           for appimage in dist/release/*.AppImage; do
             chmod +x "$appimage"
             APPIMAGE_EXTRACT_AND_RUN=1 "$appimage" --version
+            # Update information (#349) is embedded, and the .zsync for delta updates sits beside it.
+            "$appimage" --appimage-updateinformation | grep -F 'gh-releases-zsync|'
+            test -s "$appimage.zsync"
           done
       - uses: actions/upload-artifact@v4
         with: