Function primary_or_all_indices 

fn primary_or_all_indices(
    primary: Option<usize>,
    monitor_count: usize,
) -> Vec<usize>

Monitor indices to cover for the Primary placement. When the windowing system names a primary monitor we cover exactly it. When it can’t (Wayland has no “primary” concept) we cover every monitor instead of just the first: “the primary screen” is meaningless there, and leaving the other monitors uncovered lets the user dodge an enforceable break by glancing at the next screen (#67, Steffi’s dual-monitor setup). Pure so the fallback is unit-testable without a windowing system.