assets.generatePreviews

Queue a render of the missing or stale preview.png for every world asset whose type declares a preview: recipe in its type.yaml — the batch pass that brings an existing world up to the previews-carry-the-description contract. Each asset is pictured by its type's recipe (a material on a sphere, a shader on the shader ball, a texture as its own pixels, anything scene-instantiable stood in the preview studio) and the picture is persisted inside the asset folder, where it syncs, publishes, drives image-based search, and gives browsers a thumbnail. The call itself only scans and enqueues, so it returns in seconds regardless of world size; the renders drain serially on the preview work queue in the background — poll assets.previewStatus until pending reaches 0. Re-running the tool is safe at any point: assets whose preview is current, one that records the content the asset holds now, are skipped unless force re-renders them, so an interrupted backfill resumes where it left off. root narrows the pass to a path prefix. Empty leftover README.md files inside visited assets are removed — an empty file can never sync, so each one otherwise re-enters the outgoing queue on every boot.