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

changes/remove-package-size-reports.md

@@ -0,0 +1,4 @@
+type: changed
+area: release
+
+- Removed package-size JSON reports from future releases and their CI size comparisons. Package-content validation remains enabled.