ci: implement dual-version Swift testing (5.9 + 6.2)

- Add macOS jobs for Swift 5.9 (Xcode 15.2) and 6.2 (Xcode 16.2)
- Add Linux jobs for Swift 5.10 and 6.2 containers
- Update Windows CI to Swift 6.2 (already using SwiftyLab/setup-swift)
- Test both debug and release configurations on macOS
- Ensures compatibility at both ends of support range (Point-Free approach)

Author: coenttb

.github/workflows/ci.yml

@@ -14,13 +14,43 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
-  macos:
-    name: macOS (Swift 6.1+)
+  macos-swift59:
+    name: macOS (Swift 5.9)
+    runs-on: macos-14
+    strategy:
+      matrix:
+        xcode: ['15.2']
+        config: ['debug', 'release']
+    steps:
+      - uses: actions/checkout@v5
+
+      - name: Select Xcode ${{ matrix.xcode }}
+        run: sudo xcode-select -s /Applications/Xcode_${{ matrix.xcode }}.app
+
+      - name: Print Swift version
+        run: swift --version
+
+      - name: Cache Swift packages
+        uses: actions/cache@v4
+        with:
+          path: .build
+          key: ${{ runner.os }}-spm-${{ hashFiles('Package.resolved') }}
+          restore-keys: |
+            ${{ runner.os }}-spm-
+
+      - name: Build
+        run: swift build -c ${{ matrix.config }}
+
+      - name: Run tests
+        run: swift test -c ${{ matrix.config }}
+
+  macos-swift62:
+    name: macOS (Swift 6.2)
     runs-on: macos-15
     strategy:
       matrix:
-        xcode: ['16.0']
-        config: ['release']
+        xcode: ['16.2']
+        config: ['debug', 'release']
     steps:
       - uses: actions/checkout@v5
 
@@ -44,10 +74,23 @@ jobs:
       - name: Run tests
         run: swift test -c ${{ matrix.config }}
 
-  linux:
-    name: Ubuntu (Swift 6.0)
+  linux-swift510:
+    name: Ubuntu (Swift 5.10)
+    runs-on: ubuntu-latest
+    container: swift:5.10
+    steps:
+      - uses: actions/checkout@v5
+
+      - name: Build
+        run: swift build
+
+      - name: Run tests
+        run: swift test
+
+  linux-swift62:
+    name: Ubuntu (Swift 6.2)
     runs-on: ubuntu-latest
-    container: swift:6.0
+    container: swift:6.2
     steps:
       - uses: actions/checkout@v5
 
@@ -58,14 +101,14 @@ jobs:
         run: swift test
 
   windows:
-    name: Windows (Swift 6.1)
+    name: Windows (Swift 6.2)
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v5
 
       - uses: SwiftyLab/setup-swift@latest
         with:
-          swift-version: "6.1"
+          swift-version: "6.2"
 
       - name: Build
         run: swift build