feat(frontend): v2 artifact viewers for cte_plan/cte_result/phase

Add CtePlanViewer, CteResultViewer, and PhaseSummaryViewer to render the
structured v2 payloads (schema_version: 2) the harness now emits for
artifact-gate widgets, per contracts.md. Extract PreviewGrid from
ResultsPanel as a reusable AG Grid component shared by CteResultViewer.
ArtifactView dispatches to the new viewers on a schema_version/shape
guard, falling back to the existing legacy renderers unchanged for
older sessions and replay fixtures.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/viewers/PreviewGrid.test.tsx

@@ -0,0 +1,13 @@
+import { render, screen } from "@testing-library/react";
+import { PreviewGrid } from "./PreviewGrid";
+
+test("renders grid cells for the given columns and rows", () => {
+  render(
+    <PreviewGrid columns={["name", "age"]} rows={[["Alice", 30], ["Bob", 25]]} />
+  );
+
+  expect(screen.getByText("Alice")).toBeInTheDocument();
+  expect(screen.getByText("Bob")).toBeInTheDocument();
+  expect(screen.getByText("30")).toBeInTheDocument();
+  expect(screen.getByText("25")).toBeInTheDocument();
+});