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.

Start in studio-native assets/preview/codebox-client.ts and trace its dynamic startPlaygroundWeb import and use of client_module_url. Read the existing browser SDK entry points and the Codebox preview boot DTO/session boot contract. Done means a versioned browser SDK method exposes the current browser client/session handle so the consumer no longer imports @wp-playground/client directly.

Written by the indexing model from the issue text.