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/package.json

@@ -0,0 +1,37 @@
+{
+  "name": "@vue-tui/testing",
+  "version": "0.0.0",
+  "description": "Test harness for @vue-tui/runtime.",
+  "license": "MIT",
+  "files": [
+    "dist"
+  ],
+  "type": "module",
+  "exports": {
+    ".": "./dist/index.mjs",
+    "./package.json": "./package.json"
+  },
+  "publishConfig": {
+    "access": "public"
+  },
+  "scripts": {
+    "build": "vp pack",
+    "dev": "vp pack --watch",
+    "test": "vp test",
+    "check": "vp check",
+    "prepublishOnly": "vp run build"
+  },
+  "dependencies": {
+    "@vue-tui/runtime": "workspace:*"
+  },
+  "devDependencies": {
+    "@types/node": "^25.6.2",
+    "@vitejs/plugin-vue-jsx": "catalog:",
+    "typescript": "^6.0.3",
+    "vite-plus": "^0.1.20",
+    "vue": "^3.4.0"
+  },
+  "peerDependencies": {
+    "vue": "^3.4.0"
+  }
+}