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

apps/showcase_ui/src/app/components/legacy-agent-stream/legacy-agent-stream.component.html

@@ -35,7 +35,7 @@
           *ngFor="let session of agentService.sessions()"
           [value]="session.session_id"
         >
-          {{ session.initial_goal || 'Unnamed Goal' }} ({{ session.start_time * 1000 | date:'short' }})
+          {{ session.initial_goal || 'Unnamed Goal' }} ({{ session.device_serial ? session.device_serial + ' · ' : '' }}{{ session.start_time * 1000 | date:'short' }})
         </option>
       </select>
       <button class="btn-refresh" (click)="agentService.fetchSessions()">🔄 Refresh</button>