Modernize GitHub Actions for Xcode 16.4 and 26.6

[lm2343635]

Summary

This PR replaces the implicit CocoaPods-only macos-latest build with a pinned SwiftPM build matrix.

Changes

• update actions/checkout from v2 to v4
• add read-only workflow permissions
• cancel superseded builds for the same ref
• build on macOS 15 / Xcode 16.4
• build on macOS 26 / Xcode 26.6
• print the active Xcode and Swift versions
• copy the SwiftPM manifest and source target into an isolated temporary package so the legacy CocoaPods Xcode project cannot shadow package discovery
• resolve dependencies through Swift Package Manager
• build the RxController package scheme for a generic iOS Simulator destination

Why

The repository should make its supported toolchains explicit. Pinning the matrix gives maintainers a visible compatibility signal whenever GitHub changes macos-latest or Apple ships a new Xcode.

The isolated package directory is intentional: the first CI run proved that invoking xcodebuild at the repository root discovers the legacy Xcode project and fails on its missing _Pods.xcodeproj instead of testing the Swift package.

Validation

• SwiftPM dependency resolution succeeds
• Xcode 16.4 / Swift 6.1.2 build succeeds
• Xcode 26.6 build succeeds
• Generic iOS Simulator package build succeeds on both runners

Related: #16