feat(linux): ship an AppImage, with the tarball as the fallback

[StuartCameronCode]

Summary

The Linux release was a bare tarball: extract it and run ./vapourbox, with no desktop entry, no icon and no update path. package-linux.sh now also builds an AppImage and its .zsync from the same tree. The tarball stays as the fallback.

• Scripts/package-linux.sh — builds VapourBox-X.Y.Z-linux-<arch>.AppImage + .AppImage.zsync alongside the tarball. --skip-appimage gives the tarball alone.
• packaging/linux/ (new) — AppRun, the desktop entry, and a 256px icon rendered from the existing Swift source (generate-app-icons.sh regenerates it).
• build-linux.yml — installs zsync and desktop-file-utils; uploads all three files per arch.
• ci-build-and-release.sh, release.sh — the release upload picks up the AppImage and .zsync; release-notes text updated.
• packaging_test.dart — lints the update filename, the upload lists, and the desktop/icon naming against APPLICATION_ID.
• Docs — README install steps and download table, docs/BUILDING.md, a Linux section in CLAUDE.md.

Design notes

• AppRun exports nothing (no LD_LIBRARY_PATH, no bundled GTK): its environment is inherited by the worker and by the ffmpeg/vspipe it spawns from the downloaded deps bundle.
• Deps stay a first-launch download under $XDG_DATA_HOME. The AppImage mount is read-only, and deps are versioned separately in two CPU tiers.
• Update information is gh-releases-zsync|…|latest|VapourBox-*-linux-<arch>.AppImage.zsync. A missing zsyncmake fails the build rather than embedding a pointer to a file that was never made.
• appimagetool is pinned to 1.9.1 by version and SHA-256. The runtime it embeds is fetched by the tool at build time and is not pinned.

Not changed

• The glibc 2.39 floor — that comes from the deps build, not the packaging.
• An AppImage does not add itself to the menu or update itself; the README points to Gear Lever / AppImageLauncher / AppImageUpdate.
• The app window still sets no icon of its own.

Test plan

• flutter test test/packaging_test.dart passes locally (7 tests)
• The AppImage step itself has not been run — it needs a Linux host. Dispatch before merging:
  gh workflow run build-linux.yml --ref linux-appimage -f version=0.0.0-test -f deps_tag=<current deps tag> -f arch=both
• Download the artifact, chmod +x, launch on Ubuntu 24.04: deps download, a job encodes, preview renders
• desktop-file-validate clean in the build log; .zsync present in the artifact for both arches

🤖 Generated with Claude Code

https://claude.ai/code/session_014GLXdGLfPwgYjW1AkonGqN

[StuartCameronCode]

Test build of this branch is green on both architectures: https://github.com/StuartCameronCode/VapourBox/actions/runs/37301057770 (version 0.0.0-test, deps-v1.12.0).

Each artifact holds all three files — .AppImage (~13 MB), .AppImage.zsync, .tar.gz. The AppImages are static-pie ELF for the right architecture, the embedded update information reads gh-releases-zsync|StuartCameronCode|VapourBox|latest|VapourBox-*-linux-<arch>.AppImage.zsync, and the .zsync names the matching AppImage. appimagetool's only warnings are about missing AppStream metadata, which is optional.

Still unchecked: launching the AppImage on a real Linux desktop (deps download, an encode, a preview).

🤖 Generated with Claude Code