Pin CI to a runner image and Xcode with Swift 6.3

Author: colemancda

.github/workflows/ci.yml

@@ -13,27 +13,33 @@ concurrency:
 jobs:
   test:
     name: Test
-    runs-on: macos-15
+    runs-on: macos-26
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v4
+      # Pinned, not latest-stable: WasmKit 0.3.x declares swift-tools-version 6.3, so it needs
+      # Swift 6.3+. Xcode 26.5 (the macos-26 default) ships Swift 6.2.x and fails with
+      # "package 'wasmkit' is using Swift tools version 6.3.0 but the installed version is 6.2.4".
       - uses: maxim-lobanov/setup-xcode@v1
         with:
-          xcode-version: latest-stable
+          xcode-version: '26.6'
       - name: Build
         run: swift build --build-tests
       - name: Test
         run: swift test
 
   ios-archive:
     name: iOS Archive
-    runs-on: macos-15
+    runs-on: macos-26
     timeout-minutes: 60
     steps:
       - uses: actions/checkout@v4
+      # Pinned, not latest-stable: WasmKit 0.3.x declares swift-tools-version 6.3, so it needs
+      # Swift 6.3+. Xcode 26.5 (the macos-26 default) ships Swift 6.2.x and fails with
+      # "package 'wasmkit' is using Swift tools version 6.3.0 but the installed version is 6.2.4".
       - uses: maxim-lobanov/setup-xcode@v1
         with:
-          xcode-version: latest-stable
+          xcode-version: '26.6'
 
       - name: Archive
         working-directory: Darwin