Add experimental multi-monitor wallpaper sync across virtual desktops

[David-Lzy]

Summary

• add an opt-in Sync multi-monitor wallpaper across virtual desktops (experimental) tray option on Windows 11 22H2+
• compose per-display theme images into one spanned image using the monitor rectangles reported by IDesktopWallpaper
• run monitor discovery in a per-monitor-v2 DPI context, so negative coordinates, vertical layouts, mixed resolutions, and scaling are reflected in the output canvas
• assign the composite path to every virtual desktop up front instead of watching desktop switches and reapplying afterward
• restore the user's previous Center/Tile/Stretch/Fit/Fill position and per-monitor wallpapers if the feature is disabled or the virtual desktop API fails

Why

Windows 11 stores a wallpaper path for each virtual desktop, while WDD's per-display path updates only affect the active desktop. Switching desktops can therefore reveal a stale or single-monitor wallpaper until an external script reapplies the current theme.

This change gives every virtual desktop the same already-composed multi-monitor image. Switching desktops no longer needs a polling script or a delayed refresh, and the existing per-display theme choices remain visually distinct on the physical monitors.

Implementation notes

• The documented IDesktopWallpaper API supplies monitor IDs, screen-coordinate rectangles, background color, and the current wallpaper position.
• UpdateWallpaperPathForAllDesktops is an internal Windows interface. Its known Windows 11 22H2/23H2 and 24H2+ vtable layouts are isolated in one file, the option is disabled by default, and all failures are logged and rolled back.
• Unlike the virtual desktop integration removed in v5.1, this does not subscribe to virtual desktop events or add a VirtualDesktop package dependency.
• Composite files are content-addressed and cached; only the four most recent files are retained.
• Display changes, resume, and session unlock trigger a layout refresh while the option is enabled.

Tests

• dotnet build src/WinDynamicDesktop.csproj --configuration Release
• dotnet test test/WinDynamicDesktop.Tests.csproj --configuration Release --filter type!=system — 10 passed
• single-file self-contained publish succeeded for x86, x64, and ARM64
• renderer tests cover negative coordinates, vertical placement, layout gaps, per-monitor physical rectangles, Stretch, and Fit

The interactive system test was left to CI because it intentionally changes the logged-in user's real wallpaper and first-run configuration.

Closes #694
Related: #321