docs: Add GEMINI.md project summary

Author: lemmon

GEMINI.md

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+# Lemmon Validator Project Summary
+
+This project is a lightweight, fluent validation library for PHP, inspired by Valibot and Zod. It provides a structured way to validate various data types, including associative arrays, plain arrays, strings, integers, and booleans, with support for coercion, required fields, default values, and `oneOf` constraints.
+
+## Key Components:
+
+*   **`Validator.php`**: Acts as a factory for creating different validator instances (e.g., `isAssociative()`, `isArray()`, `isString()`).
+*   **`SchemaValidator.php`**: Handles validation for associative arrays, allowing definition of a schema with `FieldValidator` instances. It supports `validate()` (throws exception) and `tryValidate()` (returns status, data, errors).
+*   **`FieldValidator.php`**: An abstract base class for individual field validators. It defines common validation methods like `required()`, `default()`, `coerce()`, and `oneOf()`. Concrete validators extend this class and implement `coerceValue()` and `validateType()`.
+*   **Specific Validators (e.g., `StringValidator.php`, `IntValidator.php`, `BoolValidator.php`, `ArrayValidator.php`)**: Implement the abstract methods from `FieldValidator.php` to provide type-specific validation and coercion logic.
+*   **`ValidationException.php`**: Custom exception class used to report validation errors.
+
+## Development Setup:
+
+*   **Dependencies**: Managed by Composer (`composer.json`).
+*   **Testing**: Uses Pest PHP (`tests/ValidatorTest.php`).
+*   **Code Style**: Enforced by PHP-CS-Fixer.
+*   **Static Analysis**: Performed by PHPStan.
+*   **Local Testing**: A `temp/` directory is used for ad-hoc CLI tests, with a `_bootstrap.php` file for common setup (Composer autoloader, Symfony ErrorHandler). The `temp/` directory is excluded from Git via `.gitignore`.
+*   **Debugging**: `symfony/var-dumper` is included as a dev dependency for easy debugging.
+*   **Error Handling**: `symfony/error-handler` is included as a dev dependency and registered in the bootstrap for improved error reporting.