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/InspectorSplitHandle.module.css

@@ -0,0 +1,36 @@
+/*
+ * Horizontal splitter between the chart and the data inspector. The row-axis
+ * mirror of ResizeHandle: a 6px-tall full-width hit target with a thin centered
+ * grip, tracking the design tokens so it reads identically to the pane handles.
+ */
+.handle {
+  flex: 0 0 6px;
+  position: relative;
+  cursor: row-resize;
+  background: var(--border);
+  touch-action: none; /* let pointer drags own the gesture, not scroll */
+  transition: background var(--dur-fast) var(--ease);
+}
+
+.handle:hover,
+.handle:focus-visible {
+  background: var(--accent);
+  outline: none;
+}
+
+/* A short centered grip line (horizontal), so the handle reads as a draggable divider. */
+.grip {
+  position: absolute;
+  top: 50%;
+  left: 50%;
+  width: 24px;
+  height: 2px;
+  transform: translate(-50%, -50%);
+  background: var(--border-strong);
+  border-radius: var(--radius);
+}
+
+.handle:hover .grip,
+.handle:focus-visible .grip {
+  background: var(--accent-contrast);
+}