docs: add this pull request's own changelog fragment (#787)

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.

changelog.d/787-changelog-fragments.md

@@ -0,0 +1,7 @@
+- Changelog entries are now one file per pull request. A change adds
+  `changelog.d/<pull request>-<slug>.md` instead of editing the top of
+  `CHANGELOG.md`, which two pull requests in flight could not share — it was the
+  most frequent merge conflict in the open queue, and for six of them the only
+  one. `npm run changelog` splices the directory into `CHANGELOG.md` at release
+  time and empties it; `npm run changelog:check` reports the batch and changes
+  nothing. Fragment text is copied through unchanged (daikaginza, #787).