feat(testing): test harness for @vue-tui/runtime

Provides render() for integration testing — async with auto-flush,
trimmed frames, fake terminal with resize(), stdin.write() with
auto-flush, and auto-cleanup via afterEach.

Exports: render, cleanup, RenderResult, RenderOptions, Terminal

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/testing/README.md

@@ -0,0 +1,35 @@
+# @vue-tui/testing
+
+Test harness for [`@vue-tui/runtime`](../runtime).
+
+## Install
+
+```bash
+pnpm add -D @vue-tui/testing vue
+```
+
+## Quickstart
+
+```ts
+import { defineComponent, h } from "vue";
+import { test, expect } from "vitest";
+import { render, flush } from "@vue-tui/testing";
+import { Text } from "@vue-tui/runtime";
+
+test("renders hello", async () => {
+  const App = defineComponent({ render: () => h(Text, null, "hello") });
+  const r = render(App);
+  await flush();
+  expect(r.lastFrame()).toContain("hello");
+  r.unmount();
+});
+```
+
+## API
+
+- **`render(app, { columns?, rows? })`** — mount in a fake-TTY environment, collect frames.
+- **`flush()`** — await Vue's post-flush queue + Node's immediate queue.
+- **`result.lastFrame()` / `result.frames`** — frame snapshots.
+- **`result.stdin.write(data)`** — inject input that reaches `useInput` handlers.
+- **`result.app`** — the underlying [`TuiApp`](../runtime/README.md#tuiapp-interface) (use `.waitUntilExit()` if needed).
+- **`result.unmount()` / `result.waitUntilExit()`** — convenience pass-throughs.