Restore X11 fullscreen requests for monitor-sized borderless windows

Port the fullscreen policy omitted when adapting PR #12. Track requested fullscreen state across frame and style changes, exclude unmanaged windows, and use EWMH REMOVE when leaving fullscreen.

Extend the native geometry probe and document passive tracing for the reported vertical click offset, whose cause remains unconfirmed.

docs/TESTING.md

@@ -434,7 +434,7 @@ build/test-x11-relative-mouse
 The native probe requires the existing per-user AppKit and CoreGraphics overlays
 and a running X11/Xwayland display. It rebuilds the changed X11 and CoreGraphics
 objects and tests them in a temporary framework overlay, without installing them.
-Six `DISPLAY_GEOMETRY ...=pass` lines are required; full output is saved in
+Seven `DISPLAY_GEOMETRY ...=pass` lines are required; full output is saved in
 `build/pr12-display-check.log`.
 
 It checks the live primary screen's origin, synthetic single-monitor and offset
@@ -445,8 +445,12 @@ relative deltas, both RandR cache-invalidation events, and the root-screen fallb
 Warps are intercepted: the probe does not move the desktop pointer or send game
 input. Synthetic layouts exercise coordinate conversion and display API behavior;
 they do not simulate physical monitor hotplug or validate League rendering.
-Fullscreen policy is unchanged. Manually verify game launch, window placement,
-click alignment and edge panning on the affected monitor layout before rollout.
+The probe also checks that managed borderless windows covering a whole monitor
+request EWMH fullscreen, including on secondary monitors, and clear the property
+when resized before mapping. Titled windows, unmanaged popups and windows sized
+to the panel-reduced work area are excluded. Mapped-window transitions and panel
+stacking require manual verification under KDE Plasma/Xwayland. Also verify game
+launch, window placement, click alignment and edge panning before rollout.
 
 After installing the tested X11 and CoreGraphics binaries locally, the user
 reported that everything was working (2026-10-16). Fullscreen policy was unchanged.