Create unit tests for pkg/tar-patch package

[miabbott]

The pkg/tar-patch package (~175 lines) handles patch application but has no unit tests.

Test File to Create

• pkg/tar-patch/apply_test.go

Test Coverage

• Apply() function with various delta operations
• FilesystemDataSource implementation
• Error handling (missing files, corrupted deltas)
• Edge cases (symlinks, hardlinks, sparse files)

Acceptance Criteria

• All tests passing
• Code coverage >= 80% for pkg/tar-patch

Dependencies

• Depends on Update Go version to 1.25+ #22 (Go 1.25+ update) - tests should target modern Go version