Rewrites the iOS/macOS implementation in Swift. This is an in-place rewrite with minimal changes, to minimize the chances of breakage, and to simplify review. Future refactorings can improve the Swift structure (e.g., fully adopting thread enforcement).

Fixes flutter/flutter#119104

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Prepares for: flutter/flutter#171410

I created a calendar event for January 30, 2026. Hopefully I'll still employed then 🤞 

**Version change exemption reason**, **CHANGELOG exemption reason**, **Test exemption reason**: This only adds comments. No semantic changes.

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.