Dataset: add READMEs, organize scripts, update .gitignore

- Add README.md to all 16 dataset directories with script references
- Centralize generation scripts under scripts/ with full README
- Update .gitignore to cover all data directories, keep only metadata
- Remove redundant CCPD/.gitignore (parent covers same rules)
- Remove stale scripts/test/ (254MB pipeline residue)
- Fix label parser reference: scripts are authoritative source of truth

CCPD_Random_Blue150K_Green100K/README.md

@@ -0,0 +1,9 @@
+# CCPD Random 蓝 150K + 绿 100K
+
+| 项目 | 详情 |
+|---|---|
+| 来源 | `gen_random_bg.py` 大规模随机背景合成 |
+| 规模 | 250,002 张 |
+| 训练用途 | 未在 OCR 训练中使用 |
+
+> **以脚本为准**：此数据集为生成脚本的大规模产物，当前模型训练未引用。