chore(upm): release 0.2.1 and run split on macos host runner

Bump package to 0.2.1 (maintenance/publish, no API changes) and repoint
the Publish UPM workflow from ubuntu-latest to the macos host runner.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/upm-split.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   split:
-    runs-on: ubuntu-latest
+    runs-on: macos
     permissions:
       contents: write
     steps: