Temporary PR to track all changes together

[scottmarchant]

Overview

This PR cherry-picks the full set of async runtime changes. Individual draft PRs for focused review:

• Set up package scaffolding and CI #3 Set up package scaffolding and CI (base main)
• Implement AsyncEventLoop and MultiThreadedEventLoopGroup #4 Implement AsyncEventLoop and MultiThreadedEventLoopGroup (base build/setUpInitialPackageAndCI)
• Add NIO test utilities #5 Add NIO test utilities (base feat/implementAsyncEventLoopAndMultiThreadedEventLoopGroup)
• Add AsyncEventLoop and executor tests #6 Add AsyncEventLoop and executor tests (base chore/addTestUtils)
• Add MultiThreadedEventLoopGroup tests #7 Add MultiThreadedEventLoopGroup tests (base chore/addTestUtils)
• Implement concurrency-backed NIOThreadPool #8 Implement concurrency-backed NIOThreadPool (base build/setUpInitialPackageAndCI)
• Add NIOThreadPool tests #9 Add NIOThreadPool tests (base feat/implementNIOThreadPool)
• Add EventLoopFuture parity tests #10 Add EventLoopFuture parity tests (base chore/addTestUtils)

AsyncEventLoop stack: #3 -> #4 -> #5 -> (#6, #7, #10)
NIOThreadPool stack: #3 -> #8 -> #9

Use this umbrella PR only if you need the full combined set; otherwise review the draft stack above.