Function custom_cgroup_no_ctrl_imbalance 

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

Heavy, light, and mobile workers with tasks ping-ponging to overflow cgroup.

All four cgroups persist — the MoveAllTasks ops in every Step reference cg_mobile / cg_overflow by name and the permanent cg_heavy / cg_light workers run across the whole scenario. Declaring them in the Backdrop makes that persistence explicit and keeps per-step teardown from removing any of them at a step boundary.