Issue with ORI token in partial diffusion

[jonfunk21]

_set_origin for partial diffusion previously hard-coded ori_token=None and infer_ori_strategy="com", with two consequences:

1. The user-supplied ori_token (and infer_ori_strategy) was silently dropped. Documented behavior is that the diffused-region COM ends up within ~5 A of the user's ORI token, but in partial diffusion mode any value passed by the caller had no effect on the output coordinates.

2. The default centering used the COM of the entire input (motif + diffused region) instead of the diffused-region COM. The training pipeline uses center_option=diffuse (see configs/datasets/design_base.yaml), so the model expects the diffused region's COM at the origin. Joint-COM centering puts the diffused region far from origin in a frame the model never saw at training, which biases denoising to drag the diffused region toward the motif COM. On asymmetric systems (large target, small binder), the binder visibly drifts toward the target with increasing partial_t -- consistent with the "binder winds around target" reports.

This change updates partial-diffusion centering to:

1. Skip centering for symmetric structures (unchanged).
2. Use user-supplied ori_token / infer_ori_strategy if provided.
3. Otherwise center on the diffused-region COM (matches training).