Constant SOMA_STAGE_FD 

const SOMA_STAGE_FD: RawFd = 3;

Fixed fd number rbnx exports as ROBONIX_SOMA_STAGE_FD in soma’s environment. Any fd ≥ 3 works — we pick 3 because it’s the first non-stdio slot, which keeps ls /proc/<soma>/fd readable at a glance. Soma reads the trigger line, then closes it.