Create benchmark test suite

[miabbott]

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 Create unit tests for pkg/tar-diff package #35 (pkg/tar-diff tests)
• Depends on Create unit tests for pkg/tar-patch package #36 (pkg/tar-patch tests)