Add MLX verification scripts for DFlash setup

[gbsstudio]

Summary

• add an MLX test script for Qwen3-Coder-Next + DFlash pair diagnostics
• add a draft-only check script to validate loading z-lab/Qwen3-Coder-Next-DFlash without downloading the base model

Notes

• this helps avoid accidentally triggering large base-model downloads during local setup checks

Co-Authored-By: Oz oz-agent@warp.dev