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_322K_EXP_CUT/README.md

@@ -0,0 +1,10 @@
+# CCPD 322K YOLO 裁剪
+
+| 项目 | 详情 |
+|---|---|
+| 来源 | `CCPD_322K/` 经过 YOLO 车牌检测裁剪（12%/18% 外扩） |
+| 规模 | 321,122 张 |
+| 生成脚本 | `scripts/pipeline_322k.py` 第二步 |
+| 下游脚本 | 同脚本第三步 → `CCPD_322K_EXP_CUT_TRANS/` |
+
+> **以脚本为准**：此目录为管线中间产物，不直接用于训练。