Function record_socket_write 

pub fn record_socket_write(
    backlog: usize,
    elapsed: Duration,
    bytes: Option<usize>,
)

The websocket write itself, and how many messages were already waiting when this one was taken off the queue.

Everything else between the engine and the player is now measured; this was the last gap. The outbound channel is unbounded and the writer awaits the sink, so a client that stops draining applies backpressure and every later message queues behind it. That is invisible in the handling metric, which stops at the hand-off, and it grows with seat count because a four-seat room enqueues five envelopes per decision where a two-seat room enqueues three.