Function walk_unified 

pub fn walk_unified(
    spec: &UnifiedSpec,
    on_batch: &mut dyn FnMut(ClassifiedBatch, UnifiedCounts),
    should_stop: &(dyn Fn() -> bool + Sync),
    active_dirs: Vec<Arc<Mutex<Vec<String>>>>,
    incremental: Option<Arc<IncrementalDirState>>,
)

Walk the union of all roots across types, emitting classified batches.

The callback is invoked from the main receiving thread (single-threaded), so it does not need to be Send/Sync. Batches are ~100 files each.