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

artemis/platform/base.py

@@ -0,0 +1,104 @@
+# 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.
+
+"""Platform Abstraction Layer (PAL) - Base Protocols and Types."""
+
+from enum import Enum
+from pathlib import Path
+from typing import Protocol
+
+
+class OSType(str, Enum):
+    """Operating System Type."""
+
+    LINUX = "linux"
+    MACOS = "darwin"
+    WINDOWS = "windows"
+    UNKNOWN = "unknown"
+
+
+class IPlatformPaths(Protocol):
+    """Standardized directory and path resolver for the target operating system."""
+
+    @property
+    def config_dir(self) -> Path:
+        """Directory for persistent user/app configuration files."""
+        ...
+
+    @property
+    def data_dir(self) -> Path:
+        """Directory for persistent data (traces, SQLite DBs, replays)."""
+        ...
+
+    @property
+    def cache_dir(self) -> Path:
+        """Directory for non-essential cached data."""
+        ...
+
+    @property
+    def logs_dir(self) -> Path:
+        """Directory for persistent runtime logs."""
+        ...
+
+    def temp_dir(self, subfolder: str | None = None) -> Path:
+        """Unified temporary files directory for screenshots, recordings, and scratchpads."""
+        ...
+
+    def resolve_app_dir(self) -> Path:
+        """Resolved central application root directory for backwards compatibility."""
+        ...
+
+
+class IPlatformProcess(Protocol):
+    """Platform-specific process management, signals, and stream handling."""
+
+    @property
+    def path_separator(self) -> str:
+        """Separator character used for PATH and PYTHONPATH (':' on POSIX, ';' on Windows)."""
+        ...
+
+    def terminate_process_tree(self, pid: int, timeout_seconds: float = 3.0) -> bool:
+        """Safely and recursively terminate a process and all its child processes."""
+        ...
+
+    def setup_utf8_io(self) -> None:
+        """Ensure standard I/O and process streams operate in UTF-8 mode."""
+        ...
+
+
+class IPlatform(Protocol):
+    """Unified Platform Abstraction Layer interface."""
+
+    @property
+    def os_type(self) -> OSType:
+        """Operating system type identifier."""
+        ...
+
+    @property
+    def paths(self) -> IPlatformPaths:
+        """Platform paths resolver."""
+        ...
+
+    @property
+    def process(self) -> IPlatformProcess:
+        """Platform process and execution manager."""
+        ...
+
+    def get_package_manager_name(self) -> str | None:
+        """Name of the detected preferred package manager (e.g. apt, brew, winget)."""
+        ...
+
+    def get_install_command(self, tool_name: str) -> str:
+        """Generate platform-specific installation command for a missing tool."""
+        ...