feat(perception): expose active app metadata in UI tree

tests/test_describe_screen.py

@@ -3,11 +3,12 @@ from __future__ import annotations
 from pathlib import Path
 
 from core.errors import DriverError
-from core.models import Bounds
+from core.models import ActiveApp, Bounds, Scene
 from device.manager import DeviceManager
 from driver.base import Driver
 from perception.ocr import OCRBox
 from tools.describe_screen import describe_screen
+from tests.fakes import FakeDriver
 
 PNG_10X20 = b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00\x00\x0a\x00\x00\x00\x14"
 
@@ -91,3 +92,28 @@ def test_describe_screen_degrades_to_ocr_only_when_ui_tree_unavailable() -> None
 
     assert [element.text for element in scene.elements] == ["Search"]
     assert all(element.source == "ocr" for element in scene.elements)
+
+
+def test_describe_screen_includes_live_active_app_metadata() -> None:
+    active_app = ActiveApp(
+        platform="android",
+        package="com.example.mobile",
+        activity=".MainActivity",
+    )
+    manager = DeviceManager()
+    manager.register_device("phone-1", lambda: FakeDriver(active_app=active_app))
+    manager.connect("phone-1")
+
+    scene = describe_screen(
+        "phone-1",
+        manager=manager,
+        ocr_engine=FakeOCREngine([]),  # type: ignore[arg-type]
+    )
+
+    assert scene.active_app == active_app
+    assert scene.to_dict()["app"] == {
+        "platform": "android",
+        "package": "com.example.mobile",
+        "activity": ".MainActivity",
+    }
+    assert Scene.from_dict(scene.to_dict()).active_app == active_app