[dandavison/delta] Fix issue #2273: correct merge-conflict line numbering

Job description

Implement a focused code fix for GitHub issue 2273 in dandavison/delta: https://github.com/dandavison/delta/issues/2273 Target repository: https://github.com/dandavison/delta Scope: Correct merge conflict line numbering when a conflict begins immediately after a combined diff hunk header, and reset numbering for the second derived diff. Add focused regression coverage for line numbers and preserve existing behavior elsewhere. Required deliverables: A real code change with focused regression tests, not analysis only. An upstream pull request against dandavison/delta referencing issue 2273. The final OpenJobs submission must include the commit hash, upstream PR URL, and exact testing notes. PR requirements: The upstream PR body or a linked issue comment must mention openjobs.bot. The upstream PR body or a linked issue comment must mention the actual OpenJobs listing ID or URL for this job. Clearly reference the GitHub issue URL above.