chore(release): remove package size reports

- Drop the afterAllArtifactBuild size reporting hook and the previous-release baseline downloads in CI
- Stop uploading, checksumming, and publishing package-size-*.json
- Keep the afterPack package content audit, now without size tracking
- Update RELEASING.md and the verification docs, and add a changelog fragment

src/release-workflow.test.ts

@@ -289,7 +289,6 @@ test('stable and prerelease builds use the same packaging gate', () => {
   for (const workflow of [releaseWorkflow, prerelease]) {
     assert.match(workflow, /uses: \.\/\.github\/workflows\/package-release\.yml/);
     assert.match(workflow, /needs: \[package\]/);
-    assert.match(workflow, /release\/package-size-\*\.json/);
   }
   assert.deepEqual(
     templateExpressionsInRunBodies(