Hierarchical nesting stress

Application-scale probes for nested exogenous $U$, hierarchical DAG unrolling, and the CausalTargeted cluster-robust MSM hand-off.

Quarto notebook: docs/stress/hierarchy_stress.qmd

cd docs/stress && quarto render hierarchy_stress.qmd

See also Hierarchical / nested units and the application MixedModels recipe in examples/hierarchical_mixedmodels_concordance.md.