Add composite keys / RDF benchmark (Phase 5)

- Benchmark dense/sparse key generation from _composite_keys.py
- Benchmark TMemFile runtime composite key setup
- Benchmark end-to-end RDF query with friend joins
- Integrate with run_benchmark.sh orchestrator
- Follow existing CLI/JSON patterns for baseline_utils compatibility
- Graceful skip when ROOT not available
- Fix benchmark_rdf.py for 3-tuple return from setup_tree_with_friends()

Author: miranov25