Generate a beautified rebuild of a single-file HTML page (preview only — nothing is written).

Runs the design guide (GET /design-guide) server-side and returns either an honest abstention (the page already carries design judgement) or {direction, html} to review. Applying is a separate, explicit PUT /sites/{idOrSlug} with the returned HTML. Owner-authed; single-file, non-E2E HTML pages up to 100 KB; rate-limited per user per day. Generation can take minutes, so the 200 response streams NDJSON: {"ping":true} keepalive lines, then one terminal line — the result object, or {error, message, status} on failure. Guard errors (the non-200 codes below) are plain JSON. Agents with their own inference should prefer running the design guide themselves — same pipeline, no cap.