Function nros_platform_udp_readiness_handle 

pub unsafe extern "C" fn nros_platform_udp_readiness_handle(
    sock: *const c_void,
) -> i32

phase-521 W4 (RFC-0113 D4) — the OS handle that becomes readable when a datagram arrives on sock: its file descriptor on POSIX and NuttX. An RMW session reports it as NROS_RMW_READINESS_WAITABLE, and the executor parks on it (nros_platform_wake_park_handles_us) instead of blocking in read. Returns -1 where the stack has no such handle (lwIP, NetX, a bare-metal stack); those report readiness as a Notify instead.