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

artemis/sdk/builders/task_request_builder.py

@@ -57,7 +57,7 @@ class TaskRequestCommonBuilder(BaseModel):
     def __init__(self):
         self._max_steps = RECURSION_LIMIT
         self._record_trace = True
-        self._trace_path = Path("artemis-traces")
+        self._trace_path = Path("traces")
         self._llm_output_path: Path | None = None
         self._locked_app_package: str | None = None
         self._app_path: Path | None = None