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_Blue5K_Green5K/README.md

@@ -0,0 +1,10 @@
+# CCPD Random 蓝 5K + 绿 5K
+
+| 项目 | 详情 |
+|---|---|
+| 来源 | `gen_random_bg.py` 随机背景合成 |
+| 规模 | 10,006 张（蓝 5K + 绿 5K） |
+| 下游 | `CCPD_Random_Blue5K_Green5K_EXP_CUT/` → `..._TRANS/` |
+| 训练用途 | 早期 OCR 蓝绿新能源实验，已被 322K 管线替代 |
+
+> **以脚本为准**：`gen_random_bg.py` 生成原图，后续 EXP_CUT 和 TRANS 步骤同上。