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/core/test_toolchain.py

@@ -0,0 +1,48 @@
+# 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.
+
+"""Unit tests for ToolchainResolver and binary discovery."""
+
+from pathlib import Path
+from unittest.mock import patch
+
+from artemis.toolchain import ToolchainResolver, find_adb, find_ffmpeg, find_scrcpy
+
+
+def test_toolchain_env_override():
+    """Verify explicit environment variables override tool path resolution."""
+    resolver = ToolchainResolver()
+    fake_adb = Path("/tmp/fake_adb_binary")
+    fake_adb.touch(exist_ok=True)
+
+    with patch.dict("os.environ", {"ARTEMIS_ADB_PATH": str(fake_adb)}):
+        resolved = resolver.resolve("adb", force_refresh=True)
+        assert resolved == str(fake_adb.resolve())
+
+
+def test_toolchain_helper_functions():
+    """Verify toolchain discovery helper functions return strings."""
+    adb = find_adb()
+    ffmpeg = find_ffmpeg()
+    scrcpy = find_scrcpy()
+
+    assert isinstance(adb, str) and len(adb) > 0
+    assert isinstance(ffmpeg, str) and len(ffmpeg) > 0
+    assert isinstance(scrcpy, str) and len(scrcpy) > 0
+
+
+def test_toolchain_is_installed_lookup():
+    """Verify is_installed check on known and unknown tools."""
+    resolver = ToolchainResolver()
+    assert resolver.is_installed("non_existent_tool_xyz123") is False