Add minimum auto image size policy

[ClePol]

This PR enhances FastSurfer compatibility, by introducing an optional minimum image size.

Summary

• Add an optional minimum automatic conform size policy.
• Enforce a 256-voxel minimum for FastSurfer directory workflows.
• Preserve field-of-view-based automatic sizing for standalone inpainting.
• Propagate the policy through the CLI, conformance checks, and resampling.
• Add focused CLI and geometry-policy test coverage.

Motivation

FastSurfer workflows require a minimum conformed image size of 256³. Standalone inpainting should continue to determine its output size automatically from the input field of view.

Validation

Tested in validation pipeline (together with other lit related testing)