Create RPM spec file for Fedora packaging

Create a tar-diff.spec file following Fedora Go Packaging Guidelines for submission to Fedora.

## Tasks

- Create `tar-diff.spec` following https://docs.fedoraproject.org/en-US/packaging-guidelines/Golang/
- Include main package with CLI tools (tar-diff, tar-patch)
- Include devel subpackage for Go library
- Include man pages
- Confirm debug info rpms are included
- Create `PACKAGING.md` documenting the build and packaging process for distribution packagers

## Acceptance Criteria

- Spec file passes `rpmlint`
- `rpmbuild` produces valid SRPM and RPM
- Package installs correctly with `dnf`
- PACKAGING.md provides clear instructions for packagers

## Dependencies

- Depends on #19 (make dist target)
- Depends on #40 (man pages)