Create Pipeline

Create and execute a texture generation pipeline.

Three input modes:

1. Existing session: Provide session_id (from /upload-usd).
2. File upload: Provide usd_file, creates new session.
3. S3 reference: Provide s3_uri, downloads from S3 server-side.

Optionally provide material_textures_json to specify per-material texture prompts, blend opacity, and per-prim overrides.