Use macOS-15 runner for Xcode 16.3 availability

Author: claude

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ on:
 jobs:
   test:
     name: Test
-    runs-on: macOS-14
+    runs-on: macOS-15
     steps:
       - uses: actions/checkout@v4
       - name: Xcode Select Version