fix: Add fallback Swift installation for Linux CI environment

- Add continue-on-error to swift-actions/setup-swift to handle GPG failures
- Implement manual Swift installation as fallback when action fails
- Download Swift 6.0.3 directly from official Swift releases
- Add verification step to ensure Swift is properly installed and accessible
- Improve CI reliability for Linux compatibility testing

This resolves GPG verification issues in the swift-actions/setup-swift action
that were preventing Linux CI builds from completing successfully.

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

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

Author: fatbobman

.github/workflows/linux-compatibility.yml

@@ -21,6 +21,30 @@ jobs:
       uses: swift-actions/setup-swift@v2
       with:
         swift-version: ${{ matrix.swift-version }}
+      continue-on-error: true
+      id: swift-setup
+    
+    - name: Manual Swift installation (fallback)
+      if: steps.swift-setup.outcome == 'failure'
+      run: |
+        echo "Swift setup failed, attempting manual installation..."
+        
+        # Download Swift
+        wget -q https://download.swift.org/swift-6.0.3-release/ubuntu2004/swift-6.0.3-RELEASE-ubuntu20.04.tar.gz
+        
+        # Extract Swift
+        tar xzf swift-6.0.3-RELEASE-ubuntu20.04.tar.gz
+        
+        # Add to PATH
+        echo "$PWD/swift-6.0.3-RELEASE-ubuntu20.04/usr/bin" >> $GITHUB_PATH
+        
+        # Verify installation
+        $PWD/swift-6.0.3-RELEASE-ubuntu20.04/usr/bin/swift --version
+    
+    - name: Verify Swift installation
+      run: |
+        swift --version
+        which swift
     
     - name: Cache Swift packages
       uses: actions/cache@v4