Add comprehensive CUDA 13 support effort analysis

This analysis examines the effort required to add CUDA 13.0 support
to rust-cuda with dynamic LLVM version detection and switching.

Key findings:
- CUDA 13 introduces NVVM IR 2.0 (breaking change from 1.x)
- Requires dual LLVM support: 7.0.1 (legacy) + 20.1.0 (Blackwell+)
- Estimated effort: 6-8 weeks for experienced developer
- Recommended approach: runtime backend selection by architecture
- Maintains backward compatibility with CUDA 11.2+

Analysis includes:
- Current state assessment of codebase
- CUDA 13.0 breaking changes documentation
- Detailed 5-phase implementation plan
- Risk assessment and mitigation strategies
- Comparison to PR Rust-GPU#227 (LLVM 19 effort)
- Test strategy and validation metrics

Related to: Rust-GPU#299, Rust-GPU#227

Author: claude