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_combined_5k5k_spe/README.md

@@ -0,0 +1,10 @@
+# CCPD Combined 5K+5K+SPE
+
+| 项目 | 详情 |
+|---|---|
+| 来源 | 混合 Blue 5K + Green 5K + SPE 特殊车牌 |
+| 规模 | 37,907 张 |
+| 生成脚本 | `scripts/build_ccpd_combined.py` |
+| 训练用途 | YOLO 检测模型训练 |
+
+> **以脚本为准**：`build_ccpd_combined.py` 负责从多个源目录合并并转换标注格式。