Compose a new page from an intent

The Add → Page omnibox: classify the intent to a dashboard ARCHETYPE (a PRESET page carrying type + focus, re-derived at bake) or, when it names a specific chart, compose one authored figure via the deterministic figure-intent resolve (a CUSTOM page). Returns an appendable PageV3 directly — this endpoint persists NOTHING; the FE lands the page via the canvas PUT. Synchronous (like figure-intent): one classify call + one engine resolve, no bake to run, and the editor awaits the result — a job + poll would only add latency. The blocking LLM / profile / render work runs off the event loop.