Who owns a screen's async work: .task, a Task in the model, refreshable and cancellation?

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The gist

All async work needs an owner whose lifetime matches how long the work is needed. The owner decides when to cancel it. A SwiftUI screen has three levels of ownership:

• .task / .task(id:) — the work belongs to the view's identity: it is cancelled when the view disappears or the id changes. Loading the screen's data.
• A Task stored in the model — the work belongs to the screen model: it is cancelled on an event (a new query, a "Stop" button) and tested without UI.
• An app-level service — work that must outlive the screen: sending a message, uploading a file, syncing. The screen enqueues the job in the service and observes its status.