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.

docs/MAINTAINER_WORKFLOW.md

@@ -57,7 +57,8 @@ with a reason; usefulness can also be `decline` or `discuss`.
 - Inspect removed tests, skipped tests, relaxed assertions, snapshot updates, mocks,
   and test-runner changes. A green result from weakened tests is not acceptance.
 - Check applicable documentation updates: runtime changes need `CURRENT-STATE.md`
-  and `CHANGELOG.md`; source changes need `DATA_SOURCES.md`.
+  and a `changelog.d/<pr-number>-<slug>.md` fragment rather than an edit to
+  `CHANGELOG.md`; source changes need `DATA_SOURCES.md`.
 
 ### 3. Security and supply chain
 
@@ -173,6 +174,14 @@ Before merging:
    a stale merge. Verify the merged result matches the validated candidate tree
    and record the resulting commit and PR URL. Stop and report any mismatch.
 
+Pending release notes live in `changelog.d/` as one file per pull request, so an
+integration never has to resolve the top of `CHANGELOG.md`. When cutting a release,
+run `npm run changelog:check` to see the batch and the order it would take, then
+`npm run changelog` to splice it into `CHANGELOG.md` and empty the directory. The
+assembler refuses a malformed filename or two fragments claiming one pull request
+number before it writes anything, and copies the text through unchanged, so the
+contributor's own wording and credit are what ship.
+
 Repository rules enforce only their configured requirements; this document and
 the skill do not configure branch protection or CI. Maintainers should require
 CI and review on `main` and dismiss stale approvals when changes are pushed.