Code Quality: Switch Win32 compat mode off on Linux (P4-Z)

[MemerGamer]

Resolved / Related Issues

Phase 4 work package P4-Z (docs/linux-port/PLAN.md, Phase 4 plan Revision 3). No issue.

Stacked on #73 (P4-E). P4-E wasn't merged when this started, so this branch is based on linux/p4-e. Merge #73 first; this PR's own change is the last commit (889d848).

What changed

• Directory.Build.props: FilesWin32Compat now defaults to false. -p:FilesWin32Compat=true remains as an opt-in inventory lens and now also defines FILES_WIN32_COMPAT, so files that only build in compat mode (Utils/Shell, Services/Windows) still compile.
• Errors per build iteration, compat off: 2 → 53 → 2 → 0. After IStorageItemWithPath.Item became Windows-only, one more round: 2 → 10 → 0.
• Moved to *.Windows.cs unchanged: CloudDrivesDetector, GenericCloudDetector, OXDriveCloudDetector (registry and exe icons) and MtpHelpers (Shell COM; its only caller is already under #if WINDOWS).
• Small #if WINDOWS blocks with Linux fallbacks:
  • Recycle Bin empty uses Task.Run instead of STATask.
  • Library display name, library attributes and SeerPro are Windows-only.
  • BringToForegroundEx, the ShellItem/ShellDataObject drag payload (Linux uses DesktopFileDragHelper) and GetDoubleClickTime (Linux uses 500 ms) are guarded.
  • Format drive from the network widget goes through DriveHelpers.OpenFormatDialogAsync.
• ShellHelpers.cs (portable) is compiled again on desktop.
• New stubs in DesktopStubServices.cs:
  • DesktopCommonDialogService (LINUX-TODO(pickers): xdg-desktop-portal FileChooser). Pickers report cancelled, as the Win32 version effectively did on Linux.
  • DesktopStartMenuService: no pins.
• Removed:
  • IStorageItemWithPath.Item and its explicit implementations are now #if WINDOWS. Drag-out-of-archive checks use a new IsArchiveMember() extension (Windows: the original ZipStorage* test; Linux: ArchiveFolder/ArchiveEntryFile or IsZipPath).
  • Unused desktop WinRT StorageHelpers members are deleted: ToStorageItem(IStorageItemWithPath), ToStorageItemResult, GetFileSize, ToType.
• New stubs (LINUX-TODO):
  • storage-sense: Open Storage Sense does nothing.
  • launch: commands typed into the address bar are reported as an invalid path.
• ci-linux.yml: the Files.App desktop build passes -p:FilesWin32Compat=false and is blocking. A new step fails if Files.App.CsWin32 was built. Locally, after deleting its bin/obj, the default build left both absent, and the project graph has no CsWin32 reference.
• The compat-off baseline is now 0 keys. PLAN.md marks Phase 4 done (and Phase 2's criterion met). phase2-errors.md is marked resolved/historical.

Steps used to test these changes

1. dotnet build src/Files.App -f net10.0-desktop (compat off by default): 0 errors.
2. -p:FilesWin32Compat=true: 0 errors.
3. Platform tests: 864 passed, 7 skipped, 0 failed.
4. Headless smoke test on the compat-off build (headless-run.sh, showcase drives and seed). Each step passed: browse; copy into Reports; move into Notes; trash; restore from the Recycle Bin; browse into Projects.zip and its Projects folder; live exec touch/rm; search; Properties. The files on disk were checked afterwards.
5. The Windows target can't be built on this machine, so the #if WINDOWS branches and *.Windows.cs files haven't been compiled.

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