Add CodSpeed performance benchmarking integration
This commit integrates CodSpeed for continuous performance monitoring:
- Added benchmark suite using divan framework with CodSpeed compatibility layer
- Created benchmarks for core error handling operations (creation, context, formatting, downcasting)
- Set up GitHub Actions workflow for automated benchmark runs with OIDC authentication
- Added CodSpeed badge to README
The benchmarks measure performance of key anyhow operations to detect performance regressions in CI.