Function wayland_session_from_env 

fn wayland_session_from_env(
    os: &str,
    session_type: Option<&str>,
    wayland_display: bool,
) -> bool

Whether the given OS + session env signals a Wayland session. Off Linux the Wayland env vars are irrelevant to video detection, so this is always false there. On Linux it mirrors the probe in video.rs / overlay.rs: XDG_SESSION_TYPE=wayland or a WAYLAND_DISPLAY set. Kept pure (OS + env passed in) so every branch — including the off-Linux short-circuit — is unit-testable on the single Linux coverage runner.