Consolidate threading APIs and add WebAssembly async fallbacks

[kudit]

Scope

This draft PR is intentionally separate from the TestCase/Swift Testing PR.

It begins the threading API consolidation by adding:

• an awaited, value-returning Compatibility.main overload for places currently using MainActor.run;
• cooperative asynchronous Compatibility.background fallbacks for full-runtime WebAssembly;
• an asynchronous WebAssembly sleep(seconds:) spelling while retaining the existing synchronous compatibility call for review and deprecation planning;
• matching Task.background and Task.sleep(seconds:) wrappers on WebAssembly.

Important design distinctions

• The existing main { } API remains fire-and-forget. The new await Compatibility.main { } overload waits for completion and can return a value.
• WebAssembly supports Swift async syntax in the full runtime, but a generic host does not guarantee detached threads, Dispatch, or a suspending timer. The background fallback is therefore cooperative, and sleep currently warns and returns immediately until a host timer is supplied.
• Throwing and nonthrowing awaited background work cannot necessarily collapse into one rethrows implementation when threaded hosts await Task.detached(...).value.
• Optional-returning overloads should be removable because a generic Result may itself be optional.

Follow-up work within this draft

During review, consolidate the existing duplicate implementations in Threading.swift, deprecate obsolete spellings where source compatibility warrants it, and simplify Application.track to use the shared wrappers without a broad arch(wasm32) call-site branch.

Validation required

This is a draft because the final overload set must be compiled on Apple, Linux, and WebAssembly targets before merge.