Create unit tests for pkg/tar-patch package

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 #22 (Go 1.25+ update) - tests should target modern Go version