CLI: Schema Tools

• helix viz --schema → display contract + sample for a specific renderer.
• helix schema manifest --out schemas.json → export registry.
• helix schema diff --base old.json [--target new.json] → show changes (JSON/table).
• helix schema manifest --format json (via jq) helps pin schemas in papers.

Use manifests as lockfiles for workflows or publications so data/figure contracts remain auditable.