Make screen backend selection automatic-only

[Staphylococcus]

Follow-up to #21.

Phase: v3.0.0 public configuration cleanup after the v2.x automatic-selection validation window.

Blocked by: #87 and successful v2.x field validation.

Goal

Make automatic backend selection the only normal product behavior. LG Buddy should choose the appropriate native GNOME or native Wayland provider as desktop sessions change, without asking users to select or maintain a backend.

A persistent raw config value may remain as an advanced recovery and diagnostic override, but backend selection should no longer be presented as ordinary configuration.

Scope

• Remove backend selection from fresh interactive configuration and reconfiguration.
• Remove screen.backend mutation from the public settings CLI.
• Stop presenting GNOME and Wayland as normal user-selectable modes.
• Preserve read-only reporting of the configured and resolved backend through an appropriate diagnostic surface.
• Keep screen_backend in config.env as a documented expert override for recovery, compatibility diagnosis, and unusual deployments.
• Log clearly whenever the expert override is active.
• Define upgrade behavior for existing explicit gnome or wayland values without silently losing a working configuration.
• Update settings, configurator, installer, documentation, and release-bundle coverage for the automatic-only public model.

Acceptance criteria

• Fresh and reconfigured installations always use automatic backend selection unless config.env contains an explicit expert override.
• Switching between supported GNOME and Plasma or other compatible Wayland sessions requires no LG Buddy configuration change.
• The public settings CLI cannot set or unset a screen backend.
• Users can still inspect which backend was resolved and why.
• A config.env override remains validated, observable, and effective for recovery.
• Existing explicit backend values receive a documented migration path and are not silently discarded.
• Unsupported sessions produce a precise diagnostic rather than suggesting routine backend selection.
• Tests and release documentation reflect the automatic-only public contract.

Validation gate

The v2.x window must demonstrate that:

• native GNOME and Wayland selection works across the supported desktop matrix
• compositor and service restarts recover reliably
• provider capability failures are actionable
• explicit backend selection is no longer needed for normal use

Out of scope

• Removing swayidle, tracked by Remove the swayidle compatibility backend #87 for v2.0.0.
• Removing the raw config-file recovery override.
• Changing inactivity, gamepad, screen ownership, restore policy, or logind lifecycle semantics.
• Adding a new desktop provider.