Type Alias DataStreamCallback 

pub type DataStreamCallback = Arc<dyn Fn(DataStream, ActrId) -> BoxFuture<'static, ActorResult<()>> + Send + Sync>;

Stream chunk callback type

Design Rationale

Fast path is stream-based push, not RPC, so it doesn’t need full Context:

• Only passes sender ActrId (to know where data comes from)
• Doesn’t pass Context (avoids confusing RPC and Stream semantics)
• If reverse signaling needed, user should send via OutboundGate

Aliased Type

pub struct DataStreamCallback { /* private fields */ }