Function get_child_task_id_by_tid 

pub fn get_child_task_id_by_tid(parent: TaskId, tid: Tid) -> Option<TaskId>

Resolve a direct child of parent by POSIX tid.

This remains valid for dead-but-not-yet-reaped threads because it scans the caller’s child set and the retained task object instead of relying on the global tid index removed during exit.

Lock order: GLOBAL_SCHED_STATE before SCHED_IDENTITY (see module docs).