Move X11 dodge monitor behind visibility service

[edumucelli]

Summary

• add an X11 visibility service that creates the existing WindowDodgeMonitor behind the backend interface
• wire dock window construction through the selected backend visibility service instead of instantiating the X11 monitor directly
• mark the session-backend PR complete in the Wayland migration plan and advance the active step to visibility service wiring

Notes

• X11VisibilityService.supports_hide_mode() only reports modes that need foreign-window overlap monitoring: intelligent hide, dodge active, window dodge, and dodge maximized.
• NONE, ALWAYS_ON_TOP, and AUTOHIDE remain valid hide modes, but they are handled by the existing autohide/surface paths and do not need a dodge monitor.

[netlify]

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