Function wayland_session_from_env 

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

Pure Wayland-session test over the two relevant env signals, split out so the decision is unit-testable without mutating process env. Mirrors the probes in scheduler::overlay and video; kept local so the window path stays self-contained.