docs(mobile/4): document async tasks

[simonhamp]

What

Adds a new Async Tasks page to the NativePHP Mobile v4 docs, under Digging Deeper (order: 240, between Push Notifications and Queues).

Why

Async tasks were undocumented. Without a reference page, the two things most likely to bite people are invisible:

• the work closure must be static (a non-static closure throws at dispatch time), and
• finished() / failed() callbacks are scoped to the screen that dispatched them, so they're dropped if the user navigates away.

Contents

• Overview, with the freeze-the-screen problem async tasks solve
• Why the work closure must be static, and how to pass data in via use
• finished() and failed(), including the AsyncTaskException stand-in and originalClass()
• Screen-scoped callbacks vs. shared() named events consumed with #[On]
• Running several tasks concurrently against the background context pool
• Task classes extending AsyncTask with a handle() method
• Testing with AsyncTask::fake() and its available assertions
• A How It Works section covering the background PHP context and the native event channel
• Things to Note summary

Throughout, the page draws a clear line between async tasks and queued jobs — async tasks are not durable, not retried, and don't survive the app being killed — and cross-links to the existing Queues page.

Docs only; no application code changed.

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