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.

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

@@ -40,12 +40,28 @@ 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/exploration/data-inspector-exploration.md`](../exploration/data-inspector-exploration.md).
+### The data inspector rides the boundary too
+
+The data inspector (the Live Preview and Chart Builder panel showing the chart's input
+vs. resolved rows — spec §04) reads runtime rows through the handle, never the raw view:
+`RenderHandle.inspectData()` returns the input + resolved tables (`{ input, resolved }`,
+or `null` when no chart is up), wrapping the view exactly like `toImageURL`. It works in
+two layers:
+
+- **Enumerate + pick (`view.getState` + `core/result-data`).** A compiled Vega dataflow
+  holds many named datasets; Vega-Lite names them by convention — `source_<n>` per parsed
+  source, `data_<n>` per transform stage. The pure `pickSourceDataset` / `pickResultDataset`
+  choose the **most-upstream source** (the input) and **most-downstream output** (what the
+  marks draw), skipping dataflow internals (`marks`, `root`, layout, selection `*_store`s).
+  A spec with no transforms resolves both to the same table. The picking is pure (in `core`,
+  unit-tested); only the enumeration touches the view.
+- **Read lazily.** `getState` serializes the datasets it lists, so it is **only called while
+  the panel is open** — a collapsed inspector costs nothing, which is why the panel reads on
+  demand rather than on every render.
+
+Limitation: one name per direction can't represent a multi-view spec (layer/concat/facet
+produce several `data_<n>`); the most-downstream/upstream ones are returned, and a full
+dataset selector is left as a future option.
 
 ---
 

docs/architecture/10-interaction-and-feedback.md

@@ -210,6 +210,14 @@ each above its min. **valuenow trade-off:** it reports the library's 0–100 pos
 rendered position after a window enlargement (worst near the extremes). Drag/keyboard read live
 `clientWidth`, so resizing itself stays accurate; only the announced value drifts.
 
+**Horizontal variant — the data-inspector divider.** The chart ↔ data-inspector divider in
+the Live Preview (`InspectorSplitHandle`, spec §04) is the same window-splitter contract with
+the axis flipped: `aria-orientation="horizontal"`, ↑/↓ + Home/End, sizing a stacked region's
+**height** instead of a pane's width. It reads its flanking siblings (chart above, inspector
+below) like `PaneSplitHandle`, with the gesture in `useRowResizeDrag` (the row twin of
+`useColResizeDrag`) and the clamp/value math pure in `AppStore` (`clampInspectorHeight` /
+`inspectorHeightValue`). Rendered only while the inspector is open.
+
 _(Consulted via `/council` → WAI-ARIA APG `windowsplitter`. This bullet is the contract;
 cite it, not the APG file.)_