feat: introduce MCP server, cross-platform runtime, and Showcase UI reorganization

- Showcase UI Reorganization:
  * Move web UI dashboard from root directory into apps/showcase_ui/ for cleaner project structure
  * Add 'artemis ui' CLI command (artemis/interfaces/cli/commands/ui.py) and launcher scripts for the interactive showcase console
- Standalone MCP Server & Notifiers (mcp_server/):
  * Implement dedicated MCP server package with mobile automation tools (device_state, inspect_trace, task_manager, task_runner)
  * Add multi-channel notifiers (AgentAPI, Desktop notifications, Webhooks, File output, and Composite routing)
  * Introduce background task runner and trace store for autonomous mobile execution monitoring
- Cross-Platform Runtime & Toolchain Support:
  * Add artemis/platform/ with OS-specific implementations for Linux, macOS (Darwin), and Windows
  * Add artemis/runtime/supervisor.py for resilient process and agent runtime supervision
  * Add artemis/toolchain/ with dependency resolution and SDK descriptors
  * Add cross-platform launcher scripts (start.sh, start.bat) and installer scripts (scripts/install_deps.*)
- Core Enhancements & Diagnostics:
  * Enhance coordinate normalization, OCR API fallback, and perception pipeline
  * Add credential validation (credentials_validator.py) and system diagnostics
  * Update bilingual documentation (README.md, README_CN.md) and architecture diagrams
- Test Suite Expansion:
  * Add integration tests for cloud gateway, remote ADB, and Artemis login
  * Add unit tests covering core diagnostics, platform detection, toolchain, credentials validator, optional OCR, and MCP tools

tests/unit/data_engine/test_latency_optimizations_deep.py

@@ -328,7 +328,10 @@ async def test_ocr_api_persistent_http_client_singleton_and_tls_reuse():
         ]
     }
 
-    with patch.object(client1, "post", new_callable=AsyncMock) as mock_post:
+    with (
+        patch.object(client1, "post", new_callable=AsyncMock) as mock_post,
+        patch.dict("os.environ", {"OCR_API_KEY": "mock_api_key"}),
+    ):
         mock_post.return_value = fake_ocr_resp
         result = await ocr_api.perform_ocr(screenshot_b64="ZHVtbXlfYjY0")
 
@@ -434,7 +437,10 @@ async def test_validator_pre_execution_loop_reverted_to_exact_safety_contract(
             new_callable=AsyncMock,
         ) as mock_analyze,
     ):
-        mock_get_screen.return_value = ("dummy_b64", "dummy_stem")
+        mock_get_screen.return_value = (
+            base64.b64encode(b"dummy_bytes_here").decode("utf-8"),
+            "dummy_stem",
+        )
         # 1. XML check returns failure (TARGET_OCCUPIED by 'Sign Up' button)
         mock_xml.return_value = (
             False,