[Modernization] Establish an Xcode 26 / Swift 6 compiler support baseline

Context

RxController's current package metadata predates the current Apple toolchain:

• Package.swift uses swift-tools-version: 5.0 and iOS 10.
• The podspec declares iOS 9 and Swift 5.1.
• CI is CocoaPods-only and still uses actions/checkout@v2.
• The current Rx ecosystem has moved on: RxSwift 6.10.x includes Xcode 26 / Swift 6.2 compatibility work, and RxFlow's latest release is 2.13.0.

Apple's current Xcode 26.x line supports iOS 15+ deployment targets, so a clean Xcode 26 baseline should be treated as a next-major compatibility change rather than silently changing the 1.x contract.

Proposal

Prepare an RxController 2.0 compatibility baseline:

• iOS 15+
• Xcode 16.4 and Xcode 26.6 in CI
• Swift 5 language mode compiled by current Swift 6 compilers
• Swift Package Manager as the primary integration path
• RxSwift 6.10.x and RxFlow 2.13.x
• CocoaPods retained as a legacy transition path

Acceptance criteria

• The support policy is documented in README.
• SwiftPM manifest builds with the Xcode 16.4 and 26.6 toolchains.
• CI uses pinned, reproducible Xcode versions.
• RxSwift/RxFlow compatibility is validated.
• Release notes clearly call out the iOS 15 minimum deployment target.

Start with Package.swift, the podspec, the README, and the existing CocoaPods-only CI configuration. Check the current package metadata and CI setup against the proposed Xcode 16.4 and 26.6 toolchains, then validate the stated RxSwift and RxFlow versions. Done means the support policy, reproducible CI baseline, compatibility validation, and iOS 15 release-note change are documented.