Function util_pct_from_ticks 

fn util_pct_from_ticks(
    prev_total: u64,
    prev_idle: u64,
    curr_total: u64,
    curr_idle: u64,
) -> f64

Pure math: percentage of non-idle ticks between two snapshots (0.0–100.0 before any clamping). Takes raw pre-computed totals/idles so it can be unit-tested without constructing a CpuTime (which has private fields).