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>

testdrive-jsui/static/js/tests/test.md

@@ -0,0 +1,6 @@
+# Test Document
+
+This is a test document to check if UI controls appear in edit mode.
+
+## Section 1
+Some content here.