feat: add CLAUDE.md for project guidance and testing instructions

Author: Ark4ne

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Commands
+
+### Testing
+- `vendor/bin/phpunit` - Run all tests
+- `vendor/bin/phpunit --coverage-clover coverage.xml` - Run tests with coverage
+- `vendor/bin/phpunit --configuration phpunit.php8.1.xml.dist` - Run tests for specific PHP version
+- `vendor/bin/phpunit tests/Unit` - Run only unit tests
+- `vendor/bin/phpunit tests/Feature` - Run only feature tests
+
+### Static Analysis
+- `vendor/bin/phpstan analyze` - Run PHPStan static analysis (level 6)
+
+### Dependencies
+- `composer install` - Install dependencies
+- `composer require laravel/framework ^9.0` - Install specific Laravel version for testing
+
+## Architecture
+
+This is a Laravel package that provides JSON:API compliant resource serialization. The core architecture follows these patterns:
+
+### Resource System
+- **JsonApiResource**: Main abstract class extending Laravel's JsonResource, implements JSON:API specification
+- **JsonApiCollection**: Handles collections of resources with proper JSON:API formatting
+- **Resourceable**: Interface defining resource contracts
+
+### Key Components
+
+#### Descriptors
+- **Values** (`src/Descriptors/Values`): Type descriptors for attributes (string, integer, float, date, enum, etc.)
+- **Relations** (`src/Descriptors/Relations`): Relationship descriptors (one, many)
+
+#### Resource Concerns
+- **Attributes**: Handles attribute serialization with field filtering
+- **Relationships**: Manages relationship loading and serialization with include support
+- **ConditionallyLoadsAttributes**: Laravel-style conditional attribute support
+- **Identifier**: Resource ID and type handling
+- **Links**: JSON:API links support
+- **Meta**: Meta information handling
+- **Schema**: Resource schema generation for validation
+- **ToResponse**: Response formatting
+
+#### Request Validation
+- **Rules/Includes**: Validates `include` parameter against resource schema
+- **Rules/Fields**: Validates `fields` parameter for sparse fieldsets
+
+### Key Features
+- **Include Support**: Dynamic relationship loading via `?include=` parameter
+- **Sparse Fieldsets**: Attribute filtering via `?fields[type]=` parameter  
+- **Described Notation**: Fluent API for defining attributes and relationships with type casting
+- **Laravel Compatibility**: Supports Laravel 9-12 and PHP 8.1-8.4
+
+### Configuration
+The package includes a config file (`config/jsonapi.php`) with settings for:
+- Nullable value handling
+- Date formatting
+- Float precision
+- Automatic whenHas conditions
+- Relationship loading behavior
+
+### Testing Structure
+- **Unit Tests**: Test individual components in isolation
+- **Feature Tests**: Test complete JSON:API response formatting
+- Uses Orchestra Testbench for Laravel package testing
+- SQLite in-memory database for testing