Chart builder: filter/calculate transforms, data preview, and inline expression validation

docs/architecture/05-rendering-theming-preview.md

@@ -40,6 +40,13 @@ above it is data; everything below it is a Vega `View` we own and must tear down
   `subscribe` listener), not as reactive state itself.
 - **Don't** scatter `vegaEmbed(...)` calls across components.
 
+> **Forward note — data inspector.** A planned vega-editor-style inspector (show the
+> _resolved, post-transform_ data tables, in the builder and as a togglable panel below
+> the main Live Preview) reads runtime rows via `view.data(name)`. It rides this same
+> boundary: a `RenderHandle.data()` accessor wraps the view (like `toImageURL`), so no
+> component touches the raw view. Parked, not built — see
+> [`docs/data-inspector-exploration.md`](../data-inspector-exploration.md).
+
 ---
 
 ## 2. vega-embed Integration