[preview] Replace HookDesktopThreadMouse with verified XAML input routing

[YellowNest]

Preview of the future standalone upstream PR, stacked directly on fix/win11-native-start-button-overlap (Open-Shell#2547 head).

This branch deliberately excludes the temporary Open-Shell#2554 sync commit, so the comparison contains only the actual Win11 Start input-routing work plus the latest hardening.

Expected diff shape:

• StartMenuDLL.cpp
• Win11StartButtonTap.cpp

Safety model:

• start in WH_MOUSE fallback
• probe routed XAML pointer delivery without exposing button clicks
• retire WH_MOUSE only after the route is mapped, hit-test suppression is active, Handled = true succeeds, and a routed mouse event is successfully bridged
• immediately re-arm fallback on relevant XAML topology changes or runtime routing failures
• validate routed HWND/coordinate conversion before rewriting bridge messages
• retain fallback for configurations where every native Start slot cannot be safely routed

This is an internal preview only; the real upstream PR should be created after Open-Shell#2547 lands.