Render a single MutationRecord as a header line followed by either:
a unified diff body, when the record carries mutRecSourceLines / mutRecMutatedLines / context, or
a one-line
- original/+ replacementfallback when source lines are not available (e.g. mutations whose source span wasUnhelpfulSpanand so the plugin couldn't extract any line text).
The header is operator at file:line:colStart-colEnd[ #variant],
matching the existing surviving-mutation report so reviewers see a
single consistent format whether they look at the runtime report or the
per-module manifest .txt.