M02-L04 exercise — Stage and commit a useful change

Start: M02-L03 with the reviewed README edit still unstaged.

Stage only the reviewed README label edit, inspect the cached diff, and commit it with a message explaining its purpose. Submit the two latest history entries and git show --stat HEAD, plus a note describing what the new commit contains.

In a separate harmless README edit after this commit, stage one sentence, edit it again, and inspect both diffs. Explain which version would be committed. Resolve that experiment deliberately before continuing; preserve any wording you want to keep. Do not upload anything in this lesson.