Prefer native Wayland monitoring and deprecate swayidle

[Staphylococcus]

Parent: #21

Phase: v1.4.0 promotion and compatibility migration.

Blocked by: #85

Goal

Promote the field-validated native Wayland provider to the normal non-GNOME path while retaining delegated swayidle as an explicit, deprecated compatibility backend for existing installations and older compositors.

Scope

• Update screen_backend=auto selection to prefer:
  1. the complete native GNOME contract
  2. the supported native Wayland protocol contract
  3. delegated swayidle compatibility when installed
• Stop presenting swayidle as a normal choice in fresh interactive configuration.
• Continue accepting and running existing explicit screen_backend=swayidle configurations during the compatibility window.
• Emit a clear deprecation notice for explicit delegated-backend use.
• Do not silently rewrite an existing backend selection.
• Make the resolved provider, capability/version limitation, and fallback reason visible in settings and diagnostics.
• Document compositor requirements, compatibility fallback behavior, and the planned v2.0.0 removal.
• Extend settings, detection, installer, and release-smoke coverage for promotion and legacy configuration.

Acceptance criteria

• A supported non-GNOME Wayland session resolves to the native Wayland backend under auto.
• GNOME remains preferred when its complete native contract is available.
• An existing explicit screen_backend=swayidle value continues to work and reports deprecation clearly.
• Fresh configuration no longer encourages new delegated-backend selections.
• Unsupported native sessions either use the documented compatibility fallback or report a precise unsupported-session diagnostic.
• Settings output distinguishes configured selection, resolved backend, and fallback reason.
• Upgrade and release-smoke coverage proves legacy configuration is preserved without a silent rewrite.
• Documentation identifies the compatibility window and the conditions required before final removal.

Out of scope

• Removing swayidle code, tests, or installer support.
• Supporting protocol/compositor combinations rejected by Validate ext-idle-notify semantics for the shared inactivity engine #84.
• Changing GNOME, gamepad, logind, screen policy, or marker semantics.