feat: add compact JSON generation and playground mode

1. UI generation currently requires models to produce element IDs and references.
2. Adds catalog-constrained nested JSON generation with incremental compilation and a Compact playground mode with responsive previews.
3. No linked issue or PR.

Playground demo

Live Compact generation, output views, responsive preview, form interaction, and JSONL/YAML controls. Pauses between interactions are trimmed; generation plays at its recorded speed.

https://github.com/user-attachments/assets/a505a103-15fc-4158-8950-f30d63c55ebb