fix(changelog): make every surface split a release the same way

The app, the website and the release scripts each parse CHANGELOG.md with
their own code, and only the app skipped fenced code blocks and read CRLF.
A `###` or a release heading inside a fenced example in a highlight would
have ended the highlights early on the website and in the GitHub release
body, or started a phantom release, while the app showed it correctly.

The website and script parsers now follow the app's rules, and one case
file, scripts/test-support/changelog-structure.json, is run by all three
suites so a rule cannot change in one place only.

The release-notes script tests were not part of any CI job; they now run
with the other script contract tests.

scripts/changelog-utils.test.mjs

@@ -0,0 +1,41 @@
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import test from "node:test";
+import { parseChangelogEntries, splitChangelogHighlights } from "./changelog-utils.mjs";
+
+// The cases the app, the website and the release scripts must all split the same way.
+const structureCases = JSON.parse(
+  readFileSync(new URL("./test-support/changelog-structure.json", import.meta.url), "utf8"),
+);
+
+for (const lineEnding of ["\n", "\r\n"]) {
+  test(`splits the shared cases with ${JSON.stringify(lineEnding)} line endings`, () => {
+    const entries = parseChangelogEntries(structureCases.document.join(lineEnding));
+
+    assert.deepEqual(
+      entries.map((entry) => {
+        const { highlightLines, sectionLines } = splitChangelogHighlights(entry.bodyLines);
+        return {
+          version: entry.version,
+          date: entry.date,
+          highlights: highlightLines,
+          sections: sectionLines,
+        };
+      }),
+      structureCases.releases.map((release) => ({
+        version: release.version,
+        date: release.date,
+        highlights: release.highlights,
+        sections: release.sections,
+      })),
+    );
+  });
+}
+
+test("finds an entry for every release heading line in the repository's CHANGELOG.md", () => {
+  for (const file of ["CHANGELOG.md", "CHANGELOG.zh-CN.md"]) {
+    const changelog = readFileSync(new URL(`../${file}`, import.meta.url), "utf8");
+    const headingLines = changelog.split("\n").filter((line) => line.startsWith("## "));
+    assert.equal(parseChangelogEntries(changelog).length, headingLines.length, file);
+  }
+});