Data inspector: input/resolved rows below the chart, with a resizable divider

A collapsible Data panel under the Live Preview and Chart Builder charts shows the rows the chart actually uses, switching between Input (parsed source) and Resolved (post-transform) views read from the live Vega view. Collapsed by default; the open-state and a draggable height divider persist.

Rows come through a new RenderHandle.inspectData() accessor, so no component touches the view: core/result-data picks the most-upstream source and most-downstream result from the compiled dataflow, read lazily. The divider reuses the window-splitter pattern (horizontal variant).

Consolidations: a shared DataTable primitive replaces the inspector's and the builder's duplicate read-only tables; useResizeDrag merges the col/row drag-gesture twins.

Docs: spec 04/06 and arch 05/10 updated; the now-shipped exploration memo removed.

src/app/components/Onboarding.test.tsx

@@ -18,7 +18,7 @@ import { Onboarding } from './Onboarding';
 // never touches vega-embed. A resolved no-op handle is enough — Onboarding only
 // finalizes it on unmount.
 vi.mock('../services/chart-renderer', () => ({
-  renderSpec: () => Promise.resolve({ destroy() {}, resize() {} }),
+  renderSpec: () => Promise.resolve({ destroy() {}, resize() {}, inspectData: () => null }),
 }));
 
 (globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true;