feat: add AppImage distribution for Linux (amd64 + arm64)

[laradji]

Summary

• Add reusable composite action (.github/actions/build-appimage/) that repackages per-platform Linux tarballs into AppImage bundles using pinned appimagetool v1.9.1
• Extend release workflow with an appimage job (runs after smoke) that builds, smoke-tests (-version + ORT bootstrap), and uploads AppImages for both amd64 and arm64
• Include AppImage artifacts in the GitHub release and unified checksums.txt
• Add AppImage (Linux) install section to README.md with download/run one-liner and FUSE v2 troubleshooting

Details

• AppRun wrapper sets DEADZONE_ORT_CACHE to $XDG_CACHE_HOME/deadzone/ort so the ORT runtime cache lands outside the read-only SquashFS mount
• Smoke tests mirror the existing tarball smoke contract: -version proves the binary launches inside the FUSE mount, consolidate with empty artifacts exercises the ORT bootstrap path
• Icon: 256x256 placeholder PNG at .github/assets/deadzone.png satisfies AppImage validation
• Pipeline ordering: build → smoke → appimage → release

Test plan

• Trigger release workflow via workflow_dispatch on this branch (dry-run path)
• Verify AppImage artifacts appear for both linux/amd64 and linux/arm64
• Download an AppImage, chmod +x, run -version and consolidate locally
• Confirm checksums.txt covers both .tar.gz and .AppImage files
• Verify --appimage-extract-and-run fallback works on a FUSE-less container

Fixes #124