Function commit_attach_if_needed 

fn commit_attach_if_needed(
    holder: &mut WorkerHolder,
    method: &str,
    result: &Value,
)

If the just-succeeded call was attach_program, commit the new active program + canonicalizer into the holder WHILE THE SLOT LOCK IS STILL HELD (R2 seat-C atomicity). A parse failure leaves the prior attachment intact.