Add comprehensive README documentation for Extension framework

Author: Copilot

src/c#/GeneralUpdate.Extension/README.md

@@ -0,0 +1,197 @@
+# GeneralUpdate Extension Framework
+
+A comprehensive plugin manager framework for WPF/Avalonia desktop applications, inspired by VS Code Extension Mechanism and VSIX design.
+
+## Overview
+
+This framework provides a complete plugin/extension system with support for:
+- Multi-language runtime support (C#, Lua, Python, Node.js, native executables)
+- Advanced update mechanisms (full, incremental, differential, rollback)
+- Enterprise-level features (offline installation, repository mirrors, security policies)
+- Comprehensive UI extension points
+- Strong security and sandboxing
+
+## Architecture
+
+The framework is organized into the following namespaces:
+
+### MyApp.Extensions (Core)
+Core models and interfaces for extension management.
+
+**Key Types:**
+- `ExtensionManifest` - Extension metadata and configuration
+- `ExtensionState` - Extension lifecycle states (Installed, Enabled, Disabled, etc.)
+- `SemVersion` - Semantic versioning support
+- `ExtensionDependency` - Dependency management
+- `ExtensionPermission` - Permission system
+
+**Key Interfaces:**
+- `IExtensionManager` - Install, uninstall, enable, disable, update extensions
+- `IExtensionLoader` - Load and activate extensions
+- `IExtensionRepository` - Query and download extensions
+- `IExtensionLifecycle` - Extension lifecycle hooks
+- `IExtensionDependencyResolver` - Dependency resolution
+- `ISignatureValidator` - Package signature validation
+- `IExtensionProcessHost` - Process isolation
+- `IExtensionSandbox` - Permission and resource isolation
+
+### MyApp.Extensions.Packaging
+VSIX-inspired package specification and structure.
+
+**Key Types:**
+- `PackageManifest` - Package metadata with runtime config, dependencies, permissions
+- `PackageFileEntry` - File indexing within packages
+- `PackageSignature` - Digital signatures and certificates
+- `PackageFormatVersion` - Standardized package versioning
+
+**Package Structure Convention:**
+```
+extension-package/
+├── manifest.json
+├── assets/
+├── runtime/
+├── patches/
+└── signature/
+```
+
+### MyApp.Extensions.Updates
+Advanced update mechanisms for extensions.
+
+**Key Types:**
+- `UpdateChannel` - Stable/PreRelease/Dev channels
+- `UpdateMetadata` - Available updates and compatibility
+- `UpdatePackageInfo` - Full/Delta/Diff package details
+- `DeltaPatchInfo` - Incremental update information
+- `RollbackInfo` - Rollback configuration
+
+**Key Interfaces:**
+- `IUpdateService` - Check for updates, download, install, rollback
+- `IDeltaUpdateService` - Generate and apply delta patches
+
+### MyApp.Extensions.Runtime
+Multi-language runtime support for extensions.
+
+**Key Types:**
+- `RuntimeType` - Enum: DotNet, Lua, Python, Node, Exe, Custom
+- `RuntimeEnvironmentInfo` - Runtime configuration and environment
+
+**Key Interfaces:**
+- `IRuntimeHost` - Start/stop runtime, invoke methods, health checks
+- `IRuntimeResolver` - Resolve runtime hosts by type
+
+### MyApp.Extensions.UI
+UI extension points for WPF/Avalonia applications.
+
+**Key Interfaces:**
+- `ICommandContribution` - Register commands
+- `IMenuContribution` - Add menu items
+- `IPanelContribution` - Contribute panels/views
+- `IShortcutContribution` - Keyboard shortcuts
+- `IThemeContribution` - Custom themes
+
+**Key Types:**
+- `UIContributionManifest` - Unified UI contribution metadata
+
+### MyApp.Extensions.Security
+Enterprise-level security and offline support.
+
+**Key Types:**
+- `EnterprisePolicy` - Allowed/blocked sources, certificate requirements
+- `OfflinePackageIndex` - Local package management
+
+**Key Interfaces:**
+- `IRepositoryMirror` - Enterprise repository mirror management
+- `IOfflineInstaller` - Offline installation support
+
+### MyApp.Extensions.SDK
+Developer-facing APIs for extension authors.
+
+**Key Types:**
+- `ExtensionActivationEvent` - Trigger-based activation (onCommand, onLanguage, onView, etc.)
+
+**Key Interfaces:**
+- `IExtensionContext` - Extension runtime context and storage
+- `IExtensionAPI` - Host application services and commands
+
+## Usage Example
+
+```csharp
+// Install an extension
+var manager = GetService<IExtensionManager>();
+await manager.InstallAsync("path/to/extension.vsix");
+
+// Check for updates
+var updateService = GetService<IUpdateService>();
+var updateMetadata = await updateService.CheckForUpdatesAsync("extension-id");
+
+// Load and activate extension
+var loader = GetService<IExtensionLoader>();
+var manifest = await loader.LoadAsync("path/to/extension");
+await loader.ActivateAsync(manifest.Id);
+
+// Runtime support
+var resolver = GetService<IRuntimeResolver>();
+var pythonHost = resolver.Resolve(RuntimeType.Python);
+await pythonHost.StartAsync(runtimeEnv);
+var result = await pythonHost.InvokeAsync("main", args);
+```
+
+## Extension Development
+
+Extensions can be developed in any supported language and must include a `manifest.json`:
+
+```json
+{
+  "id": "my-extension",
+  "name": "My Extension",
+  "version": "1.0.0",
+  "author": "Author Name",
+  "runtime": "python",
+  "entrypoint": "main.py",
+  "dependencies": [],
+  "permissions": [
+    {
+      "type": "FileSystem",
+      "scope": "read",
+      "reason": "Read configuration files"
+    }
+  ],
+  "activationEvents": [
+    "onCommand:myextension.command",
+    "onStartup"
+  ]
+}
+```
+
+## Extension Activation Events
+
+Similar to VS Code, extensions can be activated based on:
+- `onStartup` - On application startup
+- `onCommand:commandId` - When a command is invoked
+- `onLanguage:languageId` - When a language file is opened
+- `onView:viewId` - When a view is opened
+- `onFileSystem:pattern` - When a file pattern is accessed
+
+## Security
+
+The framework includes comprehensive security features:
+- Digital signature validation
+- Certificate chain verification
+- Permission-based sandbox
+- Enterprise policy enforcement
+- Trusted source management
+
+## Enterprise Features
+
+- **Repository Mirrors**: Set up internal mirrors for extension repositories
+- **Offline Installation**: Deploy extensions without internet access
+- **Policy Enforcement**: Control which extensions can be installed
+- **Approval Workflows**: Require approval before installation
+
+## License
+
+This framework is part of the GeneralUpdate project and follows the same licensing terms.
+
+## Contributing
+
+Contributions are welcome! Please refer to the main GeneralUpdate repository for contribution guidelines.