Dogfoods the convention rather than describing it: the entry for #787 arrives
as `changelog.d/787-changelog-fragments.md` and `CHANGELOG.md` is untouched.
A fragment needs the pull request number, which only exists once the pull
request is open, so it lands in a second commit. `changelog.d/README.md` says
to push first and add the fragment after, for the same reason.
Checked against the one consumer that constrains the design: assembling this
fragment moves the `DISPLAY ▸ Draw` marker in `CHANGELOG.md` by exactly the
length of the spliced entry, and the 1200-character window that
`src/annotations/drawTool.test.mjs` slices after it is byte-identical.
