Function diff_fragment 

fn diff_fragment(
    prev_fragment_info: &InflightFragmentInfo,
    curr_actors: &HashMap<ActorId, InflightActorInfo>,
    upstream_fragments: HashMap<FragmentId, DispatcherType>,
    downstream_fragments: HashMap<FragmentId, DispatcherType>,
    all_actor_dispatchers: HashMap<ActorId, Vec<PbDispatcher>>,
    job_extra_info: Option<&StreamingJobExtraInfo>,
) -> MetaResult<Reschedule>

Build a Reschedule by diffing the previously materialized fragment state against the newly rendered actor layout.

This function assumes a full rebuild (no kept actors) and produces:

• actor additions/removals and vnode bitmap updates
• dispatcher updates for upstream/downstream fragments
• updated split assignments for source actors

upstream_fragments/downstream_fragments describe neighbor fragments and dispatcher types, while all_actor_dispatchers contains the new dispatcher list for each actor. job_extra_info supplies job-level context for building new actors.