Function verify 

pub fn verify(file_set: &Par2FileSet, dir: &Path) -> VerifyResult

Verify all files in a PAR2 set against actual files in a directory.

For each file described in the PAR2 set:

• If the file exists and its MD5 matches → intact
• If the file exists but MD5 doesn’t match → damaged (with per-block detail)
• If the file doesn’t exist → missing