Create benchmark test suite

Create benchmarks to establish performance baselines and track regressions.

## Tasks

- Benchmark diff generation for various file sizes
- Benchmark patch application
- Benchmark rolling checksum computation
- Benchmark bsdiff for different similarity levels
- Add `make benchmark` target

## Acceptance Criteria

- Benchmark suite covers critical code paths
- Performance baseline documented
- CI can optionally run benchmarks

## Dependencies

- Depends on #35 (pkg/tar-diff tests)
- Depends on #36 (pkg/tar-patch tests)