Add X11 window service facade

[edumucelli]

Summary

• add an X11 backend window-service facade backed by the existing Wnck WindowTracker
• expose backend-neutral WindowSnapshot data while keeping current XID/Wnck compatibility methods alive
• fold in the former PR 3 terrain prep by publishing WindowId values alongside existing xid/xids running state
• preserve latest running aggregates for service callers
• expand docs/WAYLAND.md PR sections with concrete next-session implementation direction and renumber the sequence after folding PR 3 into PR 2
• add focused X11 adapter and running-state tests for title, active, urgent, geometry, workspace, XID identity, WindowId identity, and action result mapping

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