Complete X11 session backend shape

[edumucelli]

Summary

• add an explicit backend selection entry point that currently selects the X11 session backend lazily
• add X11SessionBackend to group the current X11 window service and X11 preview service, with transitional surface/visibility services for later PRs
• pass window and preview services from the selected backend through app startup into the dock window factory
• mark PR 6 as merged in docs/WAYLAND.md and update the active migration note for PR 7

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