feat(slides): add layout density lint for sparse/empty containers

[BD-ZERO]

Description：

Summary

• 新增 xml_layout_density_lint.py 静态检查脚本，识别大型 rect
  容器内可见内容面积偏低的候选区域（大卡片配少量文字、空图片占位、单字符伪主视觉、目录卡内容过空等场景），输出可复算的几何事实（target/rule/measurement/elements），不做视觉主观判断
• 补充对应单元测试 xml_layout_density_lint_test.py
• 更新 validation-checklist.md，明确脚本路径解析方式（相对已加载 SKILL.md 的父目录，不猜测全局安装路径）及调用顺序：xml-get → xml_text_overlap_lint（结构） → xml_layout_density_lint（密度） → 截图视觉验收

Test plan

• python3 skills/lark-slides/scripts/xml_layout_density_lint_test.py 通过
• 对已知稀疏容器样例 XML 跑 xml_layout_density_lint.py，确认 sparse_container_content 命中
• 对正常密度页面跑同一脚本，确认无误报

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• Create PR with unit tests
• Commit unit tests in branch fix/slides_lint_empty

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