Main Window Desktop Layer

Requirements

Add a developer option named EnableMainWindowDesktopLayer.
When enabled, the main Lan Mountain desktop window behaves like a desktop-surface window: ordinary application windows can stay above it.
The feature is implemented as desktop-layer or bottom placement, not as Topmost.
The option is mutually exclusive with EnableFusedDesktop.
Enabling main-window desktop layer while fused desktop is enabled must ask for confirmation, then disable fused desktop on confirm or roll back on cancel.
Enabling fused desktop while main-window desktop layer is enabled must ask for confirmation, then disable main-window desktop layer on confirm or roll back on cancel.
Air APP windows remain ordinary application windows and must not be attached to the desktop layer.
On Windows, the main window should attach to the desktop icon host when available and fall back to HWND_BOTTOM when unavailable.
On non-Windows platforms, the setting may exist but the layer service is a no-op and must not throw.

Opening another app above Lan Mountain Desktop keeps that app visible when main-window desktop layer is enabled.
Restoring the main window from tray keeps the desktop-layer behavior and does not perform a temporary Topmost promotion.
Turning the option off restores normal main-window behavior as far as possible.
Fused desktop component windows keep their existing bottom-most behavior and remain isolated from the main-window service.