Function tree_admit_pid 

pub(crate) fn tree_admit_pid(counts: &mut HashMap<u32, u32>, pid: u32) -> bool

Admit one sample of pid into the window under the per-pid cap. Returns true when the sample is admitted and the pid’s count is incremented, false when the pid already holds its MLFQ_TREE_PER_PID_CAP share and the caller must drop the sample. Entries are pruned on eviction, not here: a pid at the cap keeps its entry until the window ages its samples out.