feat(ROOT): in-memory composite index for read-only files (Phase 3.1)

- Clone subframes to TMemFile when file is read-only
- Create auxiliary key tree in memory for main tree composite keys
- Use TTreeFormula to evaluate index columns (supports aliases)
- Full TTree::Draw compatibility maintained

Known issue: Test 7 (index assertion) fails due to test checking wrong
friend tree. Semantic verification passes. Will fix test later.

Author: miranov25