Function custom_fan_out_wake 

pub fn custom_fan_out_wake(ctx: &Ctx<'_>) -> Result<AssertResult>

1:N fan-out wake pattern.

One cgroup runs FutexFanOut workers: each group has 1 messenger that does CPU work then wakes 4 receivers via FUTEX_WAKE. Receivers measure wake-to-run latency. A second cgroup runs SpinWait workers to create CPU contention. Checks wake latency CV to catch inconsistent receiver placement.