Fix Wan DMD guidance anchor

[zjn20030811]

Summary

• anchor the Wan DMD real-score target at the unconditional prediction
• add a checkpoint-free regression test for the guidance target

Why

The previous expression started from the conditional prediction and then added the full guidance delta. This introduced an extra conditional term and shifted the teacher target for every guidance scale.

Validation

• pytest -q
• python -m py_compile pipelines/wan/dmd_core.py tests/test_dmd_cfg.py

[shaohao011]

Thanks !