Per-dispatch handler context.

One Ctx is built by Raxol.AgentClientProtocol.Connection for every inbound request/notification and handed to the handler task that runs the matching callback OUTSIDE the Connection process. handler_state is read-only per dispatch: handlers are concurrent tasks, so there is no safe way to thread mutable state through them — stateful per-session logic lives in the Session layer, not here.

reply_ref is present (reference()) for requests and nil for notifications. rx_seq is the inbound frame's monotone sequence stamp — begin_prompt forwards it to the Session so cancel-vs-prompt commutes by wire order. delivery_stamp is the receiver-assigned ordering key (Raxol.AgentClientProtocol.Delivery.t()) for a session/update dispatch — nil for every other dispatch.