This makes the module tree more reflective of module use patterns. In
particular, it restores the condition that every top-level module is
used in top-level code, which was broken by the addition of the
`test_assembly_chooser` module in commit 91e4e1f.
This test assembly reveals one way that the engine can stall, indicated
by a long plateau in the loss history. You can make the plateau even
longer by shrinking the inner spheres.
This avoids redundant realizations as we set an assembly's regulators
during loading. Adding some regulators to the low-curvature assembly
confirms that the feature is working as intended.
