feat: add YAML validation with check command and auto-validation

Implements comprehensive validation before PPTX conversion to catch errors early. Includes element-level validation (colors, fonts, table consistency) and system-level validation (geometry, resources). Supports standalone check command and automatic validation during conversion.

validators/__init__.py

@@ -0,0 +1,10 @@
+"""
+验证器模块
+
+提供 YAML 文件验证功能。
+"""
+
+from validators.validator import Validator
+from validators.result import ValidationResult, ValidationIssue
+
+__all__ = ['Validator', 'ValidationResult', 'ValidationIssue']