add BEFT as a new SFT/PEFT method

**Is your feature request related to a problem? Please describe.**
I initially want to add our recently published **BEFT** [(paper)](https://arxiv.org/pdf/2509.15974v2) (ACL2026 Main Conference) as **a `sft/peft` method in Tunix**. **However**, I found that the current tunix-LoRA is provided from [qwix](https://github.com/google/qwix), as shown [here](https://github.com/google/tunix/blob/main/examples/qlora_gemma.ipynb). Moreover, in `peft_trainer` ([link](https://github.com/google/tunix/blob/e0b05530b834aa971ad9c7a15beeda1dc3de422e/tunix/sft/peft_trainer.py#L211)), the lora option is provided from [flax](https://github.com/google/tunix/blob/e0b05530b834aa971ad9c7a15beeda1dc3de422e/tunix/sft/utils.py#L74).

Therefore, I am wondering that where is the best place for contributing other `sft/peft ` methods?

**Describe the solution you'd like**
Alternatively, will Tunix provide the direct interface for intergrating more `sft/peft` methods?

**Additional context**
I wish get some guidance. Thank you in advance @tianshub @s-noghabi

**Checklist**

- [x] I have searched the existing issues for similar feature requests.
- [x] This is not a support question (please use the "bug template" for that).