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/engine/reactive_runner.py

@@ -59,7 +59,7 @@ class ReactiveRunner(BaseRunner):
                     artemis_ctx = ArtemisContext(
                         device=DeviceContext(
                             host_platform=platform.os_type.name,
-                            device_serial=self.driver.device_id,
+                            device_id=self.driver.device_id,
                         )
                     )
                 flash_runner = FlashRunner(