Prefer native Wayland monitoring and deprecate swayidle

[Staphylococcus]

Summary

• resolve screen.backend=auto through GNOME, native Wayland, then deprecated swayidle compatibility
• retain and reuse the verified Wayland connection while keeping session notifications alive during backend retries
• preserve explicit swayidle configurations with deprecation diagnostics, and update configuration, installer, documentation, and smoke coverage

Validation

• cargo fmt --all --check
• cargo clippy -p lg-buddy --all-targets -- -D warnings
• cargo test -p lg-buddy --lib (735 passed, 1 ignored)
• affected Cucumber matrix (15 scenarios, 141 steps)
• release-bundle build/install/upgrade/uninstall smoke
• shell syntax checks and git diff --check

Closes #86