feat(RDF): Add modular RDataFrame API

New functions for interactive Python workflows:
- setup_rdf_with_friends() - returns (rdf, file_handle)
- setup_chain_with_friends() - TChain for multiple files
- add_defines_to_rdf() - applies Define() chain
- get_join_columns_for_snapshot() - index columns helper
- cache_to_snapshot() - convenience function

Design decisions:
- Returns tuple (rdf, file_handle) - user manages lifetime
- DRY: reuses setup_tree_with_friends()
- Immutable: add_defines returns new RDF

11 new tests, all 46 RDF tests passing

Co-authored-by: Claude (Architect)
Reviewed-by: GPT

Author: miranov25