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/IMPLEMENTATION-PLAN.md

@@ -520,8 +520,9 @@ is in the maintained plan, not the archive.
   a starter-example front door. Distinct from the shipped onboarding gallery
   (`core/examples.ts` → `Onboarding.tsx`), which seeds a snippet straight into the editor, not
   the builder — there is no spec→builder-config path yet.
-- **Transform-aware data inspector** — show resolved post-transform rows
-  (`exploration/data-inspector-exploration.md`).
+- ~~**Transform-aware data inspector** — show resolved post-transform rows~~ ✅ shipped: an
+  Input | Resolved data inspector below the Live Preview and Chart Builder charts, with a
+  draggable height divider (spec §04; arch 05 → "data inspector rides the boundary").
 - **Field-chip drag-and-drop** — click/keyboard-first shipped; drag deferred.
 - **Calculated-field autocomplete popup** — Monaco-style completion for expressions.