Fix CI: downgrade upload/download-artifact to v3 for Gitea compatibility

All 8 packages built and verified successfully in task 96 — the only
remaining failure was the very last step: actions/upload-artifact@v4
errored with GHESNotSupportedError. Gitea Actions' built-in artifact
storage doesn't implement the newer v2 upload/download API that
upload-artifact@v4/download-artifact@v4 require. Downgraded both (they
must match — v3 and v4 artifacts aren't cross-compatible) to v3, which
uses the older protocol Gitea does support.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -38,7 +38,7 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Download built packages
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v3
         with:
           name: ${{ needs.build.outputs.artifact-name }}
           path: dist