Function write_primary_bitmap 

pub fn write_primary_bitmap(
    skel: &mut BpfSkel<'_>,
    plan: &CapacityPlan,
) -> Result<()>

Phase 2 (post-load): write the primary (big-core) membership bitmap.

With uniform capacity the BPF selector short-circuits on mlfq_primary_all and never reads the map, so it is left empty. On hybrid systems a failure here leaves the map empty: the selector then treats every CPU as non-primary and falls back to any idle CPU.