feat(benchmarks): Add RDataFrame benchmark with validation

- benchmark_rdf.py: Compare RDataFrame vs TTree::Draw vs AliasDataFrame
- generate_synthetic_data.py --rdf: Generate test data with 4 subframes
- Ground truth validation using _mat suffix columns
- Integrated into run_benchmark.sh (7 benchmarks total)

Results: AliasDataFrame 25x faster than TTree::Draw at 1M rows
Validation: PASSED (RDF matches Python materialization)

Co-authored-by: Claude (Architect)

Author: miranov25