Add base_model metadata (black-forest-labs/FLUX.2-klein-4B)

This repo currently has no `base_model` set in its card metadata, so it doesn't appear in any model's tree:

```yaml
base_model:
- black-forest-labs/FLUX.2-klein-4B
```

Setting this will surface the model in black-forest-labs/FLUX.2-klein-4B's **Model Tree** (under its quantizations/finetunes) and improve discoverability for people browsing from the base model.

Feel free to merge directly (or close).

README.md

@@ -1,4 +1,6 @@
 ---
+base_model:
+  - black-forest-labs/FLUX.2-klein-4B
 license: apache-2.0
 pipeline_tag: text-to-image
 tags: