test: add comprehensive unified diff test coverage

[kharandziuk]

• Replace placeholder diff headers with proper @@ -X,Y +A,B @@ format
• Add 33 tests covering edge cases: insertions, deletions, whitespace, context mismatches, CRLF, special characters, and fuzzy matching
• Fix diff.lua to handle @@ -0,0 and @@ -N,0 insertion patterns

[deathbeam]

thanks

@all-contributors add @kharandziuk for code

[allcontributors]

@deathbeam

I've put up a pull request to add @kharandziuk! 🎉