Function snapshot_worker_allocated 

pub fn snapshot_worker_allocated(
    metrics: &PayloadMetrics,
    snap_idx: usize,
    worker_tid: i32,
) -> ThreadLookup

Extract snapshots.{snap_idx}.threads[*].allocated_bytes for the thread whose tid matches worker_tid. Returns ThreadLookup so callers distinguish “tid absent” from “cap hit before tid seen” from “allocated_bytes sibling missing” — parallel to lookup_thread, which covers the single-snapshot path.