feat: add multi-device concurrency, server lifecycle CLI, and unified execution stream

- Implement DevicePool and DeviceExecutionLock concurrency modes (per-device FIFO vs global serialization)
- Extend ArtemisClient SDK and CLI with explicit device targeting and concurrency strategy configuration
- Add server lifecycle management (artemis restart, stop, status) with cross-platform PID discovery
- Update MCP server tools to support multi-device dispatch, trace tagging, and per-device execution
- Enhance Showcase UI with step-by-step visual replay, playback speed controls, and device serial badges
- Add comprehensive unit test coverage and edge-case verification scripts

tests/unit/runtime/test_device_pool.py

@@ -0,0 +1,85 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import pytest
+
+from artemis.runtime.device_lock import DeviceExecutionLock
+from artemis.runtime.device_pool import DevicePool, DeviceStatus
+
+
+@pytest.fixture(autouse=True)
+def isolated_lock_directory(tmp_path, monkeypatch):
+    monkeypatch.setattr(
+        "artemis.runtime.device_lock.get_temp_dir",
+        lambda _name: tmp_path,
+    )
+
+
+def test_device_pool_parse_lines():
+    lines = [
+        "List of devices attached",
+        "emulator-5554          device product:sdk_gphone64_arm64 model:sdk_gphone64_arm64 device:emu64a",
+        "9A123XYZ               device product:husky model:Pixel_8_Pro device:husky",
+        "offline-dev            offline",
+        "",
+    ]
+    parsed = DevicePool._parse_device_lines(lines)
+    assert len(parsed) == 3
+    assert parsed[0] == ("emulator-5554", "device", "sdk gphone64 arm64", "sdk_gphone64_arm64")
+    assert parsed[1] == ("9A123XYZ", "device", "Pixel 8 Pro", "husky")
+    assert parsed[2] == ("offline-dev", "offline", None, None)
+
+
+def test_device_pool_list_devices_and_busy_status(monkeypatch):
+    pool = DevicePool()
+    monkeypatch.setattr(
+        pool,
+        "_query_adb_devices_sync",
+        lambda: [
+            ("emulator-5554", "device", "Emulator", "sdk"),
+            ("pixel-8-pro", "device", "Pixel 8 Pro", "husky"),
+        ],
+    )
+
+    # Initially neither is busy
+    devs = pool.list_devices()
+    assert len(devs) == 2
+    assert not devs[0].is_busy
+    assert not devs[1].is_busy
+
+    # Acquire lock for emulator-5554
+    lock = DeviceExecutionLock("emulator-5554", "test automation")
+    lock.acquire()
+    try:
+        devs = pool.list_devices()
+        emu = next(d for d in devs if d.serial == "emulator-5554")
+        pixel = next(d for d in devs if d.serial == "pixel-8-pro")
+
+        assert emu.is_busy is True
+        assert emu.active_task_desc == "test automation"
+        assert pixel.is_busy is False
+
+        # Test idle device filtering
+        idle = pool.get_idle_devices()
+        assert len(idle) == 1
+        assert idle[0].serial == "pixel-8-pro"
+
+        # Test select_device picks the idle device automatically
+        selected = pool.select_device()
+        assert selected == "pixel-8-pro"
+
+        # Test explicit selection returns requested device
+        assert pool.select_device("emulator-5554") == "emulator-5554"
+    finally:
+        lock.release()