feat(json_assets_demo): add JSON Assets Demo for policy management

Author: dgarciasquare1

example/JSON_ASSETS_DEMO.md

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+# JSON Assets Demo
+
+This demo showcases the `initializeFromJsonAssets` method of the Flutter Policy Engine, demonstrating how to load policy configurations from JSON asset files bundled with your Flutter application.
+
+## Overview
+
+The JSON Assets Demo provides an interactive interface to:
+
+- Initialize the policy manager from a JSON asset file
+- Test role-based permissions
+- Test content access control
+- View detailed role information
+
+## Features
+
+### 1. Asset-based Policy Loading
+
+- Loads policies from `assets/policies/user_roles.json`
+- Demonstrates the `initializeFromJsonAssets` method
+- Shows real-time initialization status
+
+### 2. Interactive Testing
+
+- **Role Selection**: Choose from predefined roles (admin, manager, editor, viewer, guest)
+- **Permission Testing**: Test if a role has specific permissions
+- **Content Access Testing**: Verify content access using the `hasAccess` method
+- **Role Information**: View detailed role configuration
+
+### 3. Real-time Feedback
+
+- Visual status indicators
+- Detailed error messages
+- Test results with clear success/failure indicators
+
+## JSON Asset Format
+
+The demo uses a JSON file with the following structure:
+
+```json
+{
+  "admin": {
+    "allowedContent": [
+      "read",
+      "write",
+      "delete",
+      "manage_users",
+      "system_config",
+      "all",
+      "admin_panel",
+      "user_management",
+      "system_settings"
+    ]
+  },
+  "manager": {
+    "allowedContent": [
+      "read",
+      "write",
+      "manage_team",
+      "team_content",
+      "reports",
+      "analytics",
+      "public"
+    ]
+  }
+}
+```
+
+### Required Fields
+
+- `allowedContent`: Array of strings representing permissions and content access
+
+## Usage
+
+1. **Launch the Demo**: Select "JSON Assets Demo" from the main menu
+2. **Initialize**: The policy manager automatically initializes from the JSON asset
+3. **Test Permissions**: Select a role and permission to test
+4. **Test Content Access**: Use the `hasAccess` method to verify content access
+5. **View Role Details**: Get comprehensive information about any role
+
+## Implementation Details
+
+### Asset Configuration
+
+The JSON file is declared in `pubspec.yaml`:
+
+```yaml
+flutter:
+  assets:
+    - assets/policies/
+```
+
+### Policy Manager Initialization
+
+```dart
+final policyManager = PolicyManager();
+await policyManager.initializeFromJsonAssets('assets/policies/user_roles.json');
+```
+
+### Permission Testing
+
+```dart
+final role = policyManager.roles[roleName];
+final hasPermission = role?.allowedContent.contains(permission) ?? false;
+```
+
+### Content Access Testing
+
+```dart
+final hasAccess = policyManager.hasAccess(roleName, content);
+```
+
+## Available Roles
+
+| Role    | Permissions                                      | Content Access                                      |
+| ------- | ------------------------------------------------ | --------------------------------------------------- |
+| admin   | read, write, delete, manage_users, system_config | all, admin_panel, user_management, system_settings  |
+| manager | read, write, manage_team                         | team_content, reports, analytics, public            |
+| editor  | read, write, publish                             | content_creation, drafts, published_content, public |
+| viewer  | read                                             | public, published_content, reports                  |
+| guest   | read                                             | public                                              |
+
+## Error Handling
+
+The demo includes comprehensive error handling:
+
+- Asset loading failures
+- JSON parsing errors
+- Policy initialization errors
+- Role not found scenarios
+- Permission/content access validation
+
+## Benefits
+
+1. **External Configuration**: Policies can be updated without code changes
+2. **Asset Bundling**: JSON files are bundled with the app for offline access
+3. **Flexible Structure**: Easy to modify role permissions and content access
+4. **Runtime Testing**: Interactive testing of policy configurations
+5. **Clear Feedback**: Visual indicators and detailed error messages
+
+This demo demonstrates how to effectively use the `initializeFromJsonAssets` method for flexible, asset-based policy management in Flutter applications.

example/assets/policies/user_roles.json

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+{
+  "admin": {
+    "allowedContent": [
+      "read",
+      "write",
+      "delete",
+      "manage_users",
+      "system_config",
+      "all",
+      "admin_panel",
+      "user_management",
+      "system_settings"
+    ]
+  },
+  "manager": {
+    "allowedContent": [
+      "read",
+      "write",
+      "manage_team",
+      "team_content",
+      "reports",
+      "analytics",
+      "public"
+    ]
+  },
+  "editor": {
+    "allowedContent": [
+      "read",
+      "write",
+      "publish",
+      "content_creation",
+      "drafts",
+      "published_content",
+      "public"
+    ]
+  },
+  "viewer": {
+    "allowedContent": ["read", "public", "published_content", "reports"]
+  },
+  "guest": {
+    "allowedContent": ["read", "public"]
+  }
+}