feat: implement plugin infrastructure for rendering engines

Added comprehensive plugin system for independent JavaScript UI development:

**Plugin Infrastructure:**
- Extended existing MarkiTect plugin system with RenderingEnginePlugin base class
- Added RENDERING plugin type to PluginType enum
- Created RenderingConfig for asset management and deployment
- Implemented RenderingEngineManager for plugin discovery and lifecycle

**TestDrive JSUI Plugin:**
- Extracted JavaScript UI components to independent testdrive-jsui plugin
- Created standalone development environment (no Python required)
- Implemented compass-positioned control panels (NW, NE, E, SE)
- Added clean JSON configuration interface for Python↔JavaScript data transfer

**Asset Management:**
- Development mode: serve assets directly from plugin source directory
- Production mode: deploy to _markitect/plugins/[plugin-name]/ structure
- Configurable asset URLs and deployment strategies
- Support for external dependencies (CDN resources)

**Standalone Development:**
- testdrive-jsui/test.html for browser-based development
- Package.json with npm scripts for development server
- Complete separation of JavaScript development from Python environment
- Hot reload and standard web development workflow

**Integration Demo:**
- demo_plugin_integration.py showcasing all plugin capabilities
- Standalone, plugin discovery, production deployment examples
- Asset URL generation for different deployment modes

This enables JavaScript-first development while maintaining clean integration
with the MarkiTect Python ecosystem. Developers can now work on UI components
independently using standard web development tools and workflows.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

markitect/plugins/__init__.py

@@ -16,6 +16,11 @@ from .base import (
     ExporterPlugin,
     CommandPlugin
 )
+from .rendering import (
+    RenderingEnginePlugin,
+    RenderingConfig,
+    RenderingEngineManager
+)
 from .registry import plugin_registry
 from .decorators import register_plugin
 
@@ -29,6 +34,9 @@ __all__ = [
     'ValidatorPlugin',
     'ExporterPlugin',
     'CommandPlugin',
+    'RenderingEnginePlugin',
+    'RenderingConfig',
+    'RenderingEngineManager',
     'plugin_registry',
     'register_plugin'
 ]