Standardize CI matrix: Swift 6.0-6.2 on macOS 14-15

Test across the full range of supported Swift 6 versions:
- macOS 14: Swift 6.0, 6.1
- macOS 15: Swift 6.1, 6.2

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: RISCfuture

.github/workflows/ci.yml

@@ -16,8 +16,15 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        swift: ["6.2"]
-        os: [macos-13, macos-14, macos-15]
+        include:
+          - os: macos-14
+            swift: "6.0"
+          - os: macos-14
+            swift: "6.1"
+          - os: macos-15
+            swift: "6.1"
+          - os: macos-15
+            swift: "6.2"
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4