Function serve_introspect 

pub async fn serve_introspect(
    cell: InstanceCell,
    mailbox: Mailbox,
    receiver: PortReceiver<IntrospectMessage>,
)

Introspect task: runs on a dedicated tokio task per actor, handling IntrospectMessage by reading InstanceCell directly and replying via the actor’s [Mailbox].

The actor’s message loop never sees these messages.

Invariants exercised

Exercises S1, S2, S4, S5, S6, S11 (see module doc).