fix(contextmenu): dismiss the flyout when the owning window loses focus

[kdroidFilter]

Problem

Right-click to open the OS-looking context menu, then click into another application — the menu stays on screen. It should dismiss, like every native menu does.

Why

Each backend detects outside clicks by watching its own process only:

Backend	Outside-click source	Click in another app
macOS	real NSMenu	✅ dismisses itself
Windows	thread-local WH_MOUSE hook on the popup HWND	❌ never notified
Linux	parent window's own pointer input (TaoPopupHostLinux.registerOutsidePressListener)	❌ never notified

The Windows hook is thread-local deliberately — it replaced a SetCapture monitor that forced the popup foreground, made the parent receive WM_KILLFOCUS and flipped WindowInfo.isWindowFocused false (see the comment in nucleus_tao_windows_popup.c).

On Linux the flyout is a scene layer, and a press that never lands on our surface is never reported — X11 and Wayland alike. Wayland forbids watching another surface's clicks by design. The global XI2 root-window monitor people may think of belongs to the standalone popup host (tray popups), not to this flyout.

On top of that, no backend dismissed on a focus change with no click behind it: Alt+Tab, the taskbar, a notification stealing activation.

Fix

Window focus is the one signal every backend does deliver (WindowEvent::Focused from tao — GDK focus-out on X11, wl_keyboard.leave on Wayland). Dismiss the flyout on the focused → unfocused transition.

Two details:

• LocalWindowInfo is read from the parent scene. Inside Popup the layer publishes its own WindowInfo with isWindowFocused pinned to true.
• A leading unfocused run is dropped, so a backend that has not reported focus yet when the menu opens cannot dismiss it immediately — the flyout counterpart of the not-yet-active accessory app fixed for NSMenu in a62349e.

No native code touched. macOS is unaffected.

Test

ContextMenuFocusDismissTest covers the transition logic: dismiss on focus loss, no dismiss while focused, no dismiss on a leading unfocused run, re-dismiss after a regain.

:nucleus-application:test, ktlintCheck and detekt pass.

Not verified on screen — reproducing needs a manual click into another app, which does not automate. A Wayland session in particular is reasoning-from-code, not observed. Residual gap either way: clicking a surface that takes no keyboard focus (some layer-shell docks, the desktop background on some compositors) produces no focus-out and leaves the menu open — not a regression.