[spm] add tools for building apps using SPM (POC)

Feature: Build React Native iOS apps using **Swift Package Manager** with prebuilt
XCFrameworks, as an alternative to CocoaPods.

This is a poc for the above feature, developed in companionship with Claude.

Added: SPM support for React Native iOS using prebuilt XCFrameworks from Maven. Single entry point (setup-ios-spm.js) orchestrates codegen, autolinking, artifact download, package generation, and xcodeproj generation. Includes auto-sync build phase that re-runs autolinking when dependencies change. Deterministic xcodeproj output with all UUIDs derived from project/section/id. rn-tester configured as reference implementation with spmModules in react-native.config.js.

See packages/react-native/scripts/spm/__doc__/spm-scripts.md for details.