Behavior-preserving cleanup (95 tests green):
- reuse fastvideo.layers.visual_embedding.timestep_embedding in the DiT
- single-home COSMOS3_VIDEO_NEGATIVE_PROMPT in the lightweight presets
module; drop the unused duplicate from cosmos3_pipeline
- drop the write-only Cosmos3VisionSpec.clean_latent field + its kwarg
- hoist device = next(transformer.parameters()).device out of the per-pass
_run closure in cosmos3_get_cfg_velocity