Fix CI: upgrade to macos-15 runner for Swift 6.0 support

- Change `runs-on: macos-14` to `runs-on: macos-15` (Swift 6.0+ available)
- macos-14 only has Swift 5.10, which fails Package.swift requirements

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: charlieseay

.github/workflows/test.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   test:
-    runs-on: macos-14  # Apple Silicon runner
+    runs-on: macos-15  # Swift 6.0+ required
 
     steps:
       - uses: actions/checkout@v4