feat(editor): board shell with iframe frame and selection overlay

React Flow board (pan/zoom + frame drag are pure editor-side transforms
— zero canvas-protocol traffic, <16ms budget) hosting one iframe frame
node on the adapter origin. The frame waits for runtime.ready
(queue-drain) behind an explicit 'Starting preview…' skeleton, then
requests a single Element via render-frame (core-runner-protocol
schema) plus geometry; agent-reported geometry stays frame-local and
click reports drive store selection. The selection ring renders as an
editor-side overlay from reported geometry — chrome never inside the
customer's document (ADR-028). Stories + jsdom component tests for
board/frame/overlay; Storybook stories glob extended to
packages/editor (verified via static build).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

packages/editor/src/ui/components/selection-overlay.test.tsx

@@ -0,0 +1,31 @@
+import { describe, it, expect } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { SelectionOverlay } from "./selection-overlay";
+
+describe("SelectionOverlay", () => {
+  it("renders nothing when rect is null (no selection)", () => {
+    render(<SelectionOverlay rect={null} />);
+    expect(screen.queryByTestId("selection-overlay")).not.toBeInTheDocument();
+  });
+
+  it("positions the ring from the agent-reported rect", () => {
+    render(
+      <SelectionOverlay rect={{ x: 24, y: 12, width: 320, height: 48 }} />,
+    );
+    const overlay = screen.getByTestId("selection-overlay");
+    expect(overlay).toHaveStyle({
+      position: "absolute",
+      left: "24px",
+      top: "12px",
+      width: "320px",
+      height: "48px",
+    });
+  });
+
+  it("is chrome, not content: hidden from a11y and inert to pointers (ADR-028)", () => {
+    render(<SelectionOverlay rect={{ x: 0, y: 0, width: 10, height: 10 }} />);
+    const overlay = screen.getByTestId("selection-overlay");
+    expect(overlay).toHaveAttribute("aria-hidden", "true");
+    expect(overlay).toHaveStyle({ pointerEvents: "none" });
+  });
+});