Add awaited main and WebAssembly async threading fallbacks

[kudit]

Purpose

PR #2 improves reusable test execution and Swift Testing integration. It does not add the missing asynchronous threading APIs covered here.

This PR adds a focused companion file with:

• an awaited, value-returning Compatibility.main overload for code that currently calls MainActor.run directly;
• cooperative asynchronous Compatibility.background overloads for full-runtime WebAssembly;
• a generic nonthrowing result overload that also handles optional results without a separate duplicate API;
• a throwing result overload matching the threaded-host API shape;
• an asynchronous WebAssembly Compatibility.sleep(seconds:) spelling that reports the absence of a host timer and returns immediately;
• matching Task.background and Task.sleep(seconds:) conveniences on WebAssembly.

Design boundaries

• The existing main { } remains fire-and-forget and deployment-compatible. The new awaited overload waits for completion and can return a value.
• The synchronous, dispatch-backed, delayed, and existing threaded-host implementations remain in Threading.swift.
• WebAssembly scheduling is cooperative. These APIs do not claim detached-thread or parallel execution.
• A generic WebAssembly host does not guarantee a suspending timer, so async sleep currently warns and returns immediately rather than pretending to delay.
• Application.track keeps its existing platform-specific branch; rewriting it is not required for these APIs to be useful or correct.

Review and validation

The implementation is code-complete for review. Before merge:

• build and run tests after PR Improve reusable TestCase and Swift Testing integration #2 is merged or after this branch is updated from the final main;
• confirm Apple and Linux builds do not introduce overload ambiguity;
• run Swift Package Index/WebAssembly validation;
• resolve any branch conflict caused by PR Improve reusable TestCase and Swift Testing integration #2's overlapping availability cleanup.

This PR should remain separate from PR #2 and can be squash-merged after validation.