A client on the same elevator discarded a vertical error of any size; it now
keeps its floor's height only within 4 world units, the platform's travel
over a round trip, so a teleport or fall in height still corrects. Network
version 30: every machine now carries an elevator's riders by the same
tick's movement, simulation they must agree on. distributed_riding_elevator
(was distributed_riding_moving_elevator) is named for elevators at rest too
and looks the elevator up as a machine, as bipeds.c finds it; the source/
changes are marked port:, the original's one-tick-late carry noted as fixed,
and device_machines.c's new functions moved to its public code.
Checked: ninja linux (debug) and ninja android build; pytest tools/harness
passes (492, with a new case and negative control for the bound);
network_format.txt recorded and its test passes.