Add CodSpeed performance monitoring
This commit integrates CodSpeed for continuous performance testing:
- Added codspeed-criterion-compat v4.3.0 as a drop-in replacement for criterion
- Used dependency renaming to avoid modifying benchmark code
- Created GitHub Actions workflow for automated benchmark runs
- Added CodSpeed badge to README
- Configured OIDC authentication for secure CI integration
The existing benchmarks in oxabl_lexer now run with CodSpeed instrumentation
in CI while maintaining compatibility with local cargo bench runs.