Function merge_sorted_tail 

pub(crate) fn merge_sorted_tail(
    perm: &mut Vec<EntryId>,
    batch: &[EntryId],
    cmp: impl Fn(EntryId, EntryId) -> Ordering,
)

Merge sorted batch into perm in place: each batch element binary-searches its insertion point and the segments between insertion points move once with copy_within — O(batch·log n) comparisons + O(moved) memmove + no allocation (ADR-0008).

Old elements are never reordered, and on a sorted array the strict total order (cmp id tie-break) makes the result the unique sorted merge. Arrays ordered by size/mtime can be locally stale-sorted (in-place update_stat never repositions an entry); placement there is deterministic best-effort.