Function custom_cache_pressure_imbalance 

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

CachePressure vs SpinWait cgroups under work conservation.

One cgroup runs CachePressure workers (L1-strided RMW, cache-hot) and the other runs SpinWait workers (cache-cold). Checks throughput fairness across workers (CV <= 1.0) to catch gross placement imbalance.