Function stall_predicate 

pub fn stall_predicate(samples: &[SchedSample]) -> bool

Stall predicate: returns true when samples.len() >= STALL_WINDOW AND every consecutive pair has both nr_switches delta == 0 AND sum_exec_runtime_ns delta == 0. A window shorter than STALL_WINDOW never fires (insufficient signal).

Extracted as a free function so a unit test can exercise the predicate directly with synthetic SchedSample sequences without spinning up the polling thread.