Chart builder: SelectControl pickers, channel chooser, per-type aggregates

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

@@ -471,6 +471,10 @@ the existing view is re-measured via a `ResizeObserver`-driven event — see §8
 - **Don't** put reference resolution or fit-mode logic in the renderer — it is
   pure core logic and must be unit-testable without a DOM.
 - **Don't** mutate the input spec anywhere in the pipeline.
+- **Fit modes overwrite the spec's own sizing** (Width replaces `width` _and
+  deletes_ `height`, etc.), so a surface that lets the user set an explicit
+  width/height must pass `fitMode: 'default'` while either is set and reserve the
+  container fit for auto sizing — the Chart Builder preview does exactly this.
 
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