feat(diagnostic): add human formatter

[ofabiodev]

Summary

Adds human-readable formatting for Puff diagnostics.

Changes

• Added FormatDiagnostic for rendering diagnostic code, message, source location, source line, caret range, hints, and notes.
• Added fallback formatting for diagnostics without a file or source snippet.
• Added unit tests covering single-line spans, missing file/source fallback, and note rendering.

Testing

cd src
go test ./...

Notes

This builds on the diagnostic model from the previous PR. Source loading is still planned separately, so the formatter currently accepts source text directly.