Define segmentation interface and data contract

[AssociatedPrimeIdeal]

Goal

Create a stable interface for automatic segmentation methods and segmentation sources.

Tasks

• Define expected input data shape for segmentation backends.
• Define expected output segmentation shape, label format, and validation rules.
• Add a segmentation result object or dictionary schema, including provenance/source metadata.
• Support embedded, external, and model-generated segmentations through the same interface.
• Define backend-facing config fields so the first backend can be nnUNet without baking backend logic into the pipeline core.
• Document how segmentation connects to the existing pipeline.

Acceptance Criteria

• Segmentation methods can be added without changing the pipeline core.
• Manual/external segmentation remains supported.
• Backend-specific details remain isolated behind the segmentation interface.