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.

scripts/format-scope.json

@@ -844,5 +844,7 @@
   "src/ui/imageryBoxTool.test.mjs",
   "src/ui/recentImagery.test.mjs",
   "src/layers/recentImagery/testDoubles.mjs",
-  "scripts/qa-recent-imagery.mjs"
+  "scripts/qa-recent-imagery.mjs",
+  "scripts/assemble-changelog.mjs",
+  "src/tooling/changelogFragments.test.mjs"
 ]