Function frame_bytes 

fn frame_bytes(update: &SessionUpdate) -> usize

What a frame costs against SEAT_RING_BYTES: the bytes its own JSON would write, plus the envelope around it.

Measured, not hand-listed. A walk over the fields that can grow charged only the variants it named: a frame outside that list, and a field added to one inside it, cost a fixed weight however large it was - so a new field silently under-charged the ceiling that exists to bound memory. The count is taken through a sink that keeps nothing, so the number is the memory without a second copy of it.