Function handle_report_host_pool_count 

pub(super) fn handle_report_host_pool_count(
    state: &mut State,
    host_pool: u32,
) -> Vec<Event>

Phase B.4 follow-up — pool-only drift check. Called from spawn_pool_window where the windows dimension is mid-flight (close path hasn’t completed). Compares only the pool dimension; emits DriftDetected { kind: Pool, ... } on mismatch.