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.tsx

@@ -0,0 +1,31 @@
+import { AgGridReact } from "ag-grid-react";
+import { AllCommunityModule, ModuleRegistry } from "ag-grid-community";
+
+// Register AG Grid modules once at module load time
+ModuleRegistry.registerModules([AllCommunityModule]);
+
+interface Props {
+  columns: string[];
+  rows: unknown[][];
+}
+
+/** Reusable read-only preview grid: columns + row tuples → AG Grid. */
+export function PreviewGrid({ columns, rows }: Props) {
+  const columnDefs = columns.map((col) => ({ field: col, headerName: col }));
+
+  const rowData = rows.map((row) => {
+    const obj: Record<string, unknown> = {};
+    columns.forEach((col, i) => {
+      obj[col] = row[i];
+    });
+    return obj;
+  });
+
+  return (
+    <div className="overflow-hidden rounded-xl border border-border/70 shadow-xs">
+      <div className="ag-theme-alpine" style={{ height: 400, width: "100%" }}>
+        <AgGridReact columnDefs={columnDefs} rowData={rowData} />
+      </div>
+    </div>
+  );
+}