Function flatten_parallel_fallback_reason 

pub fn flatten_parallel_fallback_reason(
    visible_rows: usize,
    batch_size: usize,
) -> Option<String>

Morsel-parallel reserve decision for the batched flatten path.

Flatten stays on the single-partition row path in this phase: morsel parallelism over flattened batches is deferred to the columnar parallel follow-up. Returns a human-readable fallback reason for parallel_fallback_reason observability when the visible row count exceeds one batch (i.e. parallel execution could have split the work), otherwise None (single batch: no parallelism forgone).