Native RoaringBitmap Integration for Apache Fluss

Apache Fluss contains dormant RoaringBitmap aggregation infrastructure but the BITMAP type is trapped in UnsupportedKeyword making it inaccessible to users. This project promotes BITMAP to a first-class logical type wires it to the existing AggregationMergeEngine and implements five SQL functions via FlussCatalog. The centrepiece is a pushdown optimization in FlinkTableSource.applyAggregates() that intercepts BITMAP_OR_AGG queries and redirects them to the TabletServer native merge engine reducing network cost from O(N) to O(G). A working prototype with 4 passing unit tests is already available. Deliverables include type system bridge server-side wiring Flink connector pushdown integration tests performance benchmarks and a quickstart guide.