Axiom Testing Async

Covers everything you need to know about testing async code in Swift, including confirmation blocks for callbacks, MainActor isolation, and timeout control with timeLimit. This is specifically the async testing slice of the larger axiom-testing skill, so you'll want to reference swift-testing.md for the basics and ui-testing.md if you're dealing with XCUITest. The confirmation pattern with expectedCount is solid for verifying callbacks fired (or didn't fire), which beats the old XCTestExpectation approach. Most useful when you're chasing race conditions or flaky async tests, or when you need to test callbacks without falling back to sleep() hacks.