Slice 1 follow-up: component-render interaction tests (CR-1/CR-2)

- vite: add @testing-library/svelte's svelteTesting() plugin so component
  render()/mount() resolves Svelte's browser build under vitest/jsdom
- dep: @testing-library/user-event for realistic interaction tests
- app: 7 component-render tests driving App.svelte through a fake socket
  (catalog render, subscribe-on-click, unsub/sub ordering, aria-current,
  error banner, action invoke)

Verified green: svelte-check 0/0, vitest 91 passed, biome clean, vite build ok.

1 files changed, 259 insertions, 0 deletions

diff --git a/src/app/App.test.ts b/src/app/App.test.ts
new file mode 100644
index 0000000..ce37586
--- /dev/null
+++ b/src/app/App.test.ts
@@ -0,0 +1,259 @@
+import type { SurfaceServerMessage } from "@dispatch/ui-contract";
+import { render, screen } from "@testing-library/svelte";
+import userEvent from "@testing-library/user-event";
+import { describe, expect, it } from "vitest";
+import type { WebSocketLike } from "../adapters/ws";
+import App from "./App.svelte";
+import { createAppStore } from "./store.svelte";
+
+interface FakeSocket extends WebSocketLike {
+	sent: string[];
+	resolveOpen(): void;
+	feedMessage(data: SurfaceServerMessage): void;
+}
+
+function fakeSocket(): FakeSocket {
+	let onopen: (() => void) | null = null;
+	let onmessage: ((ev: { data: string }) => void) | null = null;
+	const sent: string[] = [];
+
+	const ws: FakeSocket = {
+		send(data: string) {
+			sent.push(data);
+		},
+		close() {},
+		get onopen() {
+			return onopen;
+		},
+		set onopen(fn) {
+			onopen = fn;
+		},
+		get onmessage() {
+			return onmessage;
+		},
+		set onmessage(fn) {
+			onmessage = fn;
+		},
+		get onclose() {
+			return null;
+		},
+		set onclose(_fn) {},
+		resolveOpen() {
+			onopen?.();
+		},
+		feedMessage(msg: SurfaceServerMessage) {
+			onmessage?.({ data: JSON.stringify(msg) });
+		},
+		sent,
+	};
+	return ws;
+}
+
+function sentMessages(ws: FakeSocket) {
+	return ws.sent.map((s) => JSON.parse(s));
+}
+
+describe("App component interaction tests", () => {
+	it("renders empty state when catalog is empty", () => {
+		const ws = fakeSocket();
+		const store = createAppStore({ socketFactory: () => ws });
+		ws.resolveOpen();
+
+		render(App, { props: { store } });
+
+		expect(screen.getByText("No surfaces available")).toBeInTheDocument();
+
+		store.dispose();
+	});
+
+	it("renders a catalog button per entry after a catalog message", () => {
+		const ws = fakeSocket();
+		const store = createAppStore({ socketFactory: () => ws });
+		ws.resolveOpen();
+
+		ws.feedMessage({
+			type: "catalog",
+			catalog: [
+				{ id: "s1", region: "sidebar", title: "Surface One" },
+				{ id: "s2", region: "panel", title: "Surface Two" },
+			],
+		});
+
+		render(App, { props: { store } });
+
+		const buttons = screen.getAllByRole("button");
+		expect(buttons).toHaveLength(2);
+		expect(buttons[0]).toHaveTextContent("Surface One");
+		expect(buttons[1]).toHaveTextContent("Surface Two");
+
+		store.dispose();
+	});
+
+	it("clicking a catalog entry subscribes and renders its surface", async () => {
+		const ws = fakeSocket();
+		const store = createAppStore({ socketFactory: () => ws });
+		ws.resolveOpen();
+
+		ws.feedMessage({
+			type: "catalog",
+			catalog: [{ id: "s1", region: "sidebar", title: "Surface One" }],
+		});
+
+		render(App, { props: { store } });
+
+		const user = userEvent.setup();
+		const button = screen.getByRole("button", { name: /Surface One/ });
+		ws.sent.length = 0;
+		await user.click(button);
+
+		const msgs = sentMessages(ws);
+		const subscribe = msgs.find(
+			(m: { type: string; surfaceId: string }) => m.type === "subscribe" && m.surfaceId === "s1",
+		);
+		expect(subscribe).toBeTruthy();
+
+		ws.feedMessage({
+			type: "surface",
+			spec: {
+				id: "s1",
+				region: "sidebar",
+				title: "Surface One",
+				fields: [{ kind: "stat", label: "Tokens", value: "1,234" }],
+			},
+		});
+
+		expect(await screen.findByRole("heading", { name: "Surface One" })).toBeInTheDocument();
+		expect(await screen.findByText("Tokens")).toBeInTheDocument();
+		expect(await screen.findByText("1,234")).toBeInTheDocument();
+
+		store.dispose();
+	});
+
+	it("clicking a different entry unsubscribes the previous then subscribes the new", async () => {
+		const ws = fakeSocket();
+		const store = createAppStore({ socketFactory: () => ws });
+		ws.resolveOpen();
+
+		ws.feedMessage({
+			type: "catalog",
+			catalog: [
+				{ id: "s1", region: "sidebar", title: "Surface One" },
+				{ id: "s2", region: "panel", title: "Surface Two" },
+			],
+		});
+
+		render(App, { props: { store } });
+
+		const user = userEvent.setup();
+		await user.click(screen.getByRole("button", { name: /Surface One/ }));
+		ws.sent.length = 0;
+
+		await user.click(screen.getByRole("button", { name: /Surface Two/ }));
+
+		const msgs = sentMessages(ws) as Array<{ type: string; surfaceId: string }>;
+		const unsubIdx = msgs.findIndex((m) => m.type === "unsubscribe" && m.surfaceId === "s1");
+		const subIdx = msgs.findIndex((m) => m.type === "subscribe" && m.surfaceId === "s2");
+		expect(unsubIdx).toBeGreaterThanOrEqual(0);
+		expect(subIdx).toBeGreaterThanOrEqual(0);
+		expect(unsubIdx).toBeLessThan(subIdx);
+
+		store.dispose();
+	});
+
+	it("selected catalog button reflects aria-current", async () => {
+		const ws = fakeSocket();
+		const store = createAppStore({ socketFactory: () => ws });
+		ws.resolveOpen();
+
+		ws.feedMessage({
+			type: "catalog",
+			catalog: [
+				{ id: "s1", region: "sidebar", title: "Surface One" },
+				{ id: "s2", region: "panel", title: "Surface Two" },
+			],
+		});
+
+		render(App, { props: { store } });
+
+		const user = userEvent.setup();
+		const btn1 = screen.getByRole("button", { name: /Surface One/ });
+		const btn2 = screen.getByRole("button", { name: /Surface Two/ });
+
+		await user.click(btn1);
+		expect(btn1).toHaveAttribute("aria-current", "true");
+		expect(btn2).not.toHaveAttribute("aria-current");
+
+		await user.click(btn2);
+		expect(btn2).toHaveAttribute("aria-current", "true");
+		expect(btn1).not.toHaveAttribute("aria-current");
+
+		store.dispose();
+	});
+
+	it("an error message renders the alert banner", () => {
+		const ws = fakeSocket();
+		const store = createAppStore({ socketFactory: () => ws });
+		ws.resolveOpen();
+
+		ws.feedMessage({
+			type: "error",
+			message: "Something went wrong",
+		});
+
+		render(App, { props: { store } });
+
+		const alert = screen.getByRole("alert");
+		expect(alert).toHaveTextContent("Something went wrong");
+
+		store.dispose();
+	});
+
+	it("invoking a field action sends an invoke", async () => {
+		const ws = fakeSocket();
+		const store = createAppStore({ socketFactory: () => ws });
+		ws.resolveOpen();
+
+		ws.feedMessage({
+			type: "catalog",
+			catalog: [{ id: "s1", region: "sidebar", title: "Surface One" }],
+		});
+
+		render(App, { props: { store } });
+
+		const user = userEvent.setup();
+		await user.click(screen.getByRole("button", { name: /Surface One/ }));
+
+		ws.feedMessage({
+			type: "surface",
+			spec: {
+				id: "s1",
+				region: "sidebar",
+				title: "Surface One",
+				fields: [
+					{
+						kind: "toggle",
+						label: "Dark Mode",
+						value: false,
+						action: { actionId: "toggle-dark" },
+					},
+				],
+			},
+		});
+
+		ws.sent.length = 0;
+		const checkbox = await screen.findByRole("checkbox", { name: "Dark Mode" });
+		await user.click(checkbox);
+
+		const msgs = sentMessages(ws);
+		const invoke = msgs.find(
+			(m: { type: string; surfaceId: string; actionId: string; payload: unknown }) =>
+				m.type === "invoke" &&
+				m.surfaceId === "s1" &&
+				m.actionId === "toggle-dark" &&
+				m.payload === true,
+		);
+		expect(invoke).toBeTruthy();
+
+		store.dispose();
+	});
+});