feat: update backlog with detailed Flang HLFIR analysis and create codebase map

- Analyzed 51 Fortran modules and 82 active tests - complete infrastructure ready
- Updated BACKLOG.md with precise Flang-based HLFIR implementation plan
- Created CLAUDE.md with comprehensive codebase map and module documentation
- Identified critical blocker: src/mlir_c/mlir_c_stubs.c needs real MLIR C API implementation
- Corrected dependency status: fortfront available in ../fortfront/
- Ready for Epic 8: Real MLIR integration to enable actual Fortran compilation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: krystophny