Function commit_existing_directory_with 

pub fn commit_existing_directory_with<E>(
    staging: &Path,
    canonical: &Path,
    prepare: impl FnOnce(&File, &Path) -> Result<(), E>,
    at_step: impl FnMut(DirectoryCommitStep, &Path) -> Result<()>,
) -> Result<DirectoryCommitOutcome, DirectoryCommitError<E>>

Admit, prepare and publish one existing directory without replacing another name.

prepare receives the held no-follow directory descriptor and must verify and synchronize every required child and the root before returning success. It is invoked exactly once, on staging for a fresh commit or canonical for recovery. The parent is then synchronized after publication or recovery. No temporary tree is created or removed, and no failed evidence is cleaned up. A failed post-rename sync or hook requires caller reconciliation before retrying.

The caller owns stable parent namespace, exclusive writers, complete byte verification, tree traversal limits and the meaning of a recovered tree. This is not root confinement or a lock. at_step is for deterministic failure qualification; it must not issue other publication effects.

Errors

Returns path shape/conflict/missing, caller preparation or phase-aware I/O errors.