Tidy level-stage package exports #376

Goal

Tidy the level-stage package structure now that frame execution, DAG nodes, dihedral logic, and axes helpers have grown.

Background

The levels package now contains several distinct responsibilities, including DAG orchestration, frame execution, covariance nodes, axes utilities, dihedral analysis, neighbour search, and hierarchy construction.

The package structure should be cleaned incrementally so imports remain stable and the PR stays low risk.