Surface collected column statistics to planners via ShuffleReaderExec

[coderfender]

Follow-up to #1916 (part of #1827).

#1916 collects per-column null counts on the executor and stores them aggregated on SuccessfulStage.output_column_stats in the scheduler. They are not yet consumed: ShuffleReaderExec::partition_statistics (stats_for_partition in shuffle_reader.rs) still returns column_statistics: vec![ColumnStatistics::new_unknown(); num_fields]

1. finalise_stage_internal (aqe/planner.rs) — carry output_column_stats alongside partitions.
2. resolve_shuffle_partitions (aqe/execution_plan/exchange.rs + adaptive.rs) — store stats next to partition locations.
3. BallistaAdapter::transform_children (aqe/adapter.rs) — pass into ShuffleReaderExec::try_new*.
4. ShuffleReaderExec — new field plus proto/serde changes (serialized core plan node).
5. stats_for_partition (shuffle_reader.rs) — use the carried stats instead of new_unknown().
6. Collect stats on the sort-based shuffle-write path (sort_shuffle/writer.rs, currently empty; TODO in code).
7. Collect stats on the single-partition write path (shuffle_writer.rs, currently empty; TODO in code).