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/ResizeHandle.tsx

@@ -19,7 +19,7 @@
  */
 
 import { useLayoutEffect, useRef, useState } from 'react';
-import { useColResizeDrag } from '../hooks/useColResizeDrag';
+import { useResizeDrag } from '../hooks/useResizeDrag';
 import { clampSideWidth, sideWidthValue, usePanesStore, type PaneSide } from '../stores/PanesStore';
 import styles from './ResizeHandle.module.css';
 
@@ -66,7 +66,8 @@ export function ResizeHandle({ side, label }: ResizeHandleProps) {
 
   // The left handle grows its pane as it moves right; the right handle (left
   // of the preview) shrinks the preview as it moves right.
-  const onPointerDown = useColResizeDrag(
+  const onPointerDown = useResizeDrag(
+    'x',
     () =>
       side === 'library'
         ? usePanesStore.getState().libraryWidth