ci: Add GitHub Actions workflow for iOS build and testing

[synaptode]

This PR introduces a GitHub Actions workflow to automatically build and test the PhantomSwift project on every commit and pull request to the main or master branches.

Key features of this setup:

• Runs on macos-latest environment.
• Uses maxim-lobanov/setup-xcode@v1 to ensure the latest stable Xcode is utilized.
• Executes xcodebuild commands to build and test the PhantomSwift scheme targeting the iPhone 15 simulator.
• Disables code signing (CODE_SIGN_IDENTITY="", etc.) to guarantee successful runs in standard CI environments without requiring certificates.
• Includes a fallback swift test execution to verify Swift Package Manager compatibility.

This ensures all new changes are safely compiled and verified against existing tests automatically.

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PR created automatically by Jules for task 2392271467513114478 started by @synaptode

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