Set up COPR repository for testing builds

Create a COPR repository to build and test the RPM package before submitting to Fedora.

## Tasks

- Create COPR project for tar-diff
- Configure automated builds from GitHub releases or tags
- Test installation on Fedora 43, 44, Rawhide

## Acceptance Criteria

- COPR builds succeed
- Package installable via `dnf copr enable && dnf install tar-diff`

## Dependencies

- Depends on #39 (RPM spec file)