Swift: Finalize progressive rendering behavior of child lists

> It's not uncommon for 1) an `updateComponents` message to describe a component that reference other components as its children and 2) the client hasn't heard a thing about those children yet (that often arrives in a follow-up `UpdateComponents` message that comes right after the first).
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> In this scenario, wouldn't the lookup return `nil` and every caller just drops its job of rendering this subtree because `resolveChildList` skips nil children.
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> In this scenario, our prior art implements progressive rendering, putting placeholders in place of children that have yet to arrive. I don't remember off the top of my head how much better this tends to look compared to the more classical approach of just having components pop in as they arrive, so your mileage may vary.
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_Originally posted by @andrewkolos in [#2052](https://github.com/a2ui-project/a2ui/pull/2052/changes/BASE..a2a3eb346038b0723b2289a3d03b0c2799e58654#r3639480312)_

Once the Basic Catalog and Sample App are up and running, finalize the behavior of declared but missing (or incomplete) render nodes. Perhaps with a placeholder view or perhaps pop-in.

Das Issue bezieht sich auf das Verhalten der progressiven Darstellung von untergeordneten Listen in Swift. Beginne damit, die Funktion `resolveChildList` und den Umgang mit nil-Kindern zu untersuchen. Sieh dir den Basic Catalog und die Sample App an, um den aktuellen Darstellungsablauf zu verstehen. Bestimme, ob für fehlende Render-Nodes Platzhalter oder Pop-in implementiert werden sollen, und teste das visuelle Ergebnis.

Vom Indexierungsmodell aus dem Issue-Text verfasst.