Add pages structurally (no LLM turn)

The promptless '+ add page' seam: submit one or more structural page specs ({title?, type?, focus?}) and the server applies them through the SAME apply_controls pipeline the chat add_pages path uses — each page is grounded, SEEDED (a contentless authored page gets a starter composition; a comparison page gets its compare block + faceted figures) and lands baked, never an empty tab. Appends a new draft head (optimistic concurrency via base_sequence → 409) and kicks the async bake, like the canvas save. The blocking apply runs off the event loop. Prefer this over composing a page and PUTting it raw.