Avalonia cross-platform port: Windows + macOS Context Menu app

[YodasMyDad]

Summary

• Completes the WPF → Avalonia port of ImageResize.ContextMenu. Same Explorer right-click experience on Windows, plus a native macOS build surfaced through Finder's Services menu.
• Restores the single-instance + arg-forwarding behaviour that was stripped during the POC, now behind an ISingleInstance abstraction with Windows (named pipe + mutex) and Unix (domain socket + exclusive lockfile) implementations.
• Ships a universal macOS .app + .dmg via build-macos.sh (arm64 + x86_64 merged with lipo), plus an Automator Quick Action so right-click → Services → Resize Images works in Finder. CI builds the DMG on every PR.
• Pins Tmds.DBus.Protocol to 0.92.0 to clear GHSA-xrw6-gwf8-vvr9.

Test plan

• dotnet build ImageResize.ContextMenu -c Release — clean, 0 warnings.
• dotnet test — 42/42 passing.
• Windows smoke test: launch with a sample image, launch a second instance with a different image, confirm IPC forwards over named pipe and the primary receives it (verified via log).
• macOS end-to-end: drag ImageResize.app to Applications, install Resize Images.workflow, right-click an image in Finder, confirm Services → Resize Images opens the app with the selection.
• macOS single-instance forward: open app, right-click another image, confirm it appears in the existing window (not a new one).
• First-launch Gatekeeper bypass steps from the README work (right-click → Open, or xattr -dr com.apple.quarantine).

Known gaps

• No .icns icon yet — Mac app will use the default Finder icon. Drop ImageResize.ContextMenu/macos/icon.icns in and the build script picks it up automatically.
• The document.wflow Automator file is hand-written from the reverse-engineered format. If Finder doesn't show "Resize Images" under Services after install, open it in Automator.app, hit Save, and re-commit to regenerate canonical metadata.

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