Expand CI across platforms and verify binary/source parity

[HituziANDO]

Problem

CI tests one fixed iOS Simulator destination. It does not cover macOS, Mac Catalyst, Swift Package Manager consumers, CocoaPods, strict concurrency, XCFramework generation, or parity between source and the committed binary.

Scope

• Use supported GitHub Actions versions and a resilient destination selection strategy.
• Add jobs for iOS, macOS, and Mac Catalyst.
• Build a minimal Swift Package Manager consumer and lint the podspec.
• Run complete concurrency checking.
• Regenerate and compare public API, version, slices, and privacy manifests for release validation.

Dependencies

• Make XCFramework generation fail fast and reproducible #31
• Expand automated coverage for update flows and platform-specific behavior #30

Acceptance criteria

• CI detects platform-specific source regressions.
• CI detects a stale or incomplete committed XCFramework.
• Package and CocoaPods integration failures block merging or release.
• The workflow does not depend on one obsolete simulator name.

Tests

• Validate the workflow on a pull request and document any release-only job or required cache.

Relevant files

• .github/workflows/ci.yml
• Package.swift
• SwiftyUpdateKit.podspec