Expose browser preview starter through Codebox browser SDK

Studio Native still has to dynamically import the raw Playground client module and call startPlaygroundWeb during browser preview boot. That keeps a product consumer coupled to Playground internals instead of the Codebox public browser SDK.\n\nNeeded contract:\n- A versioned browser SDK method, e.g. window.wpCodeboxBrowser.v1.startBrowserPreview(...) or equivalent, that accepts the Codebox preview boot DTO/session boot contract and returns the browser client/session handle currently returned by startPlaygroundWeb.\n- The product should not need to read client_module_url or import @wp-playground/client directly.\n\nConsumer blocker:\n- studio-native assets/preview/codebox-client.ts cannot remove the dynamic startPlaygroundWeb import until this contract exists.

Beginne in studio-native assets/preview/codebox-client.ts und verfolge den dynamischen startPlaygroundWeb-Import sowie die Verwendung von client_module_url. Lies die vorhandenen Browser-SDK-Einstiegspunkte und den Codebox-Preview-Boot-DTO-/Session-Boot-Vertrag. Erledigt ist die Aufgabe, wenn eine versionierte Browser-SDK-Methode das aktuelle Browser-Client-/Session-Handle bereitstellt, sodass der Consumer @wp-playground/client nicht mehr direkt importieren muss.

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