Create unit tests for pkg/tar-diff package

[miabbott]

The pkg/tar-diff package contains the core diff generation logic (~1,436 lines) but has no unit tests.

Test Files to Create

• pkg/tar-diff/rollsum_test.go - Rolling checksum algorithm
• pkg/tar-diff/bsdiff_test.go - Binary diff generation
• pkg/tar-diff/delta_test.go - Delta writer and operation encoding
• pkg/tar-diff/analysis_test.go - Tar file analysis
• pkg/tar-diff/diff_test.go - Main Diff() function

Acceptance Criteria

• All test files created and passing
• Code coverage >= 80% for pkg/tar-diff
• Tests complete in <30s

Dependencies

• Depends on Update Go version to 1.25+ #22 (Go 1.25+ update)