feat(tooling): one changelog file per pull request

`CONTRIBUTING.md` asks every runtime change to add an entry at the top of
`CHANGELOG.md`, so two pull requests in flight write into the same few lines and
git has nothing to merge on. Against `main` at `81eb443`, `git merge-tree` over
the 85 open pull requests GitHub reports as conflicting that aren't mine puts
`CHANGELOG.md` in the conflict set of 41 — more than any other path — and for 6
it is the only one. `docs/CURRENT-STATE.md`, the other file rule 3 names,
conflicts for 23, and merges cleanly for 23 of the 41: edits land in the section
for the area you touched, so they only collide when the areas do.

A pull request now adds `changelog.d/<pull request>-<slug>.md` instead. Distinct
filenames never conflict, and the pull request number makes them distinct for
free. `npm run changelog` splices the directory into `CHANGELOG.md` and empties
it; `npm run changelog:check` reports the batch and changes nothing.

Fragment text is copied through unchanged apart from trailing whitespace.
`src/annotations/drawTool.test.mjs` reads `CHANGELOG.md` as prose and slices a
fixed window after a heading, so an assembler that reflowed an entry could move
text across that window. Fragments are deliberately outside
`scripts/format-scope.json` for the same reason.

The assembler refuses a batch before writing anything when a filename is not
`<pull request>-<slug>.md` or two fragments claim one pull request number, and
a unit test runs that check over the real `changelog.d/`, so a fragment the
release could not assemble fails `npm test` in the pull request that adds it
rather than at release time.

Refs #781.

CONTRIBUTING.md

@@ -149,7 +149,7 @@ ownership and adoption process.
 
 1. Branch off `main`.
 2. Keep `npm run build`, `npm test`, and `npm run test:track` green and avoid new console errors, plus the [feature gate](#feature-regression-gates) for the area you touched.
-3. If you change runtime behavior, update `docs/CURRENT-STATE.md` and `CHANGELOG.md` in the same PR.
+3. If you change runtime behavior, update `docs/CURRENT-STATE.md` in the same PR, and add your changelog entry as `changelog.d/<pr-number>-<slug>.md` instead of editing `CHANGELOG.md` — see [changelog.d/README.md](changelog.d/README.md). Distinct filenames don't conflict; a release assembles the directory into `CHANGELOG.md`. A documentation claim that needs a regression test should assert against `docs/CURRENT-STATE.md` or `README.md`, which a PR still edits directly.
 4. If you add or change a data source, update [DATA_SOURCES.md](DATA_SOURCES.md) with its license and attribution. **Don't add data you don't have the right to redistribute** — fetch it at runtime instead.
 5. Describe what you changed and how you verified it (screenshots welcome for anything visual).