Constant SYS_FUTEX_WAIT 

pub const SYS_FUTEX_WAIT: usize = 303;

Sleep on a futex word. Blocks if *addr == val.

• addr: futex word address
• val: expected value
• timeout_ns: timeout in nanoseconds (0 = infinite)

Returns 0 on wakeup, -EAGAIN if value mismatch, -ETIMEDOUT on timeout.