feat(dqe): multi-node Iceberg split dispatch with round-robin assignment

- Add assignSplitsToNodes() in IcebergFragmenter for round-robin
  split-to-node assignment across data nodes
- Replace executeAndMergeSplits() with dispatchIcebergSplits() +
  mergeIcebergResults() in TransportTrinoSqlAction
- Local fast path when allLocal=true (single-node, no serialization)
- Transport dispatch via sendRequest when multi-node
- Verified on 2-node cluster: 8/10 queries PASS, 2 timeout (perf)

Author: penghuo