[codex] fix circular import in mussel.utils

[raylim]

Summary

Fixes a circular import triggered by importing mussel.datasets before mussel.cli.extract_features.

Root Cause

mussel.utils.__init__ eagerly imported symbols from mussel.utils.feature_extract. That pulled in feature extraction dependencies while other modules were still initializing, which made some import orders fail in a clean interpreter.

Changes

• Lazily exports feature-extraction symbols from mussel.utils via __getattr__.
• Preserves the existing public import surface, including from mussel.utils import aggregate_slide_features_batch.
• Adds clean-interpreter regression tests for the problematic import orders.

Validation

PYTHONPATH=/tmp/mussel-circular-import /gpfs/mskmind_ess/limr/repos/Mussel-titan-fix/.venv/bin/python -m pytest /tmp/mussel-circular-import/tests/mussel/test_import_order.py -q

Result: 3 passed