createPresenceStateDerivation

Build the signal that says what to broadcast about this client.

Curried the way a sync layer wants it: bind the identity once, then apply it to a store to get a derived instance_presence record that recomputes whenever the camera moves, the selection changes, the brush is dragged or the current page changes. Push its value at whatever rate the transport likes — the signal itself does no scheduling.

Returns null before the store has an instance record, i.e. before the editor has finished starting up.