feat: Add nested hypergraph generators (Kim et al. 2023, Barrett et al. 2025)

[jg-you]

Adds two generative models that explicitly parameterize nestedness: random_nested_hypergraph and simplicial_chung_lu_hypergraph.

Barrett et al.: Implementation of Algorithms 3 and 4 from the paper. Couldn't find much to re-use between this and chung_lu_hypergraph since the methods are fundamentally different.

Kim et al.: Implements the algorithm described in Section II here. Duplicate edges are prevented during facet generation (step 1) and after rewiring (step 3) using frozenset sets. I wasn't sure whether epsilon was fixed or could vary by order, so I made this argument a list or a float to support both cases.

Notes

• Tried to match the codebase conventions for parameter names: d for edge size, p for probability, epsilon for retention, k1/k2 for degree/size sequences.
• Seed reproducibility tests only assert same-seed determinism. There's a bug in the existing code, e.g., here, where different seeds could lead to the edges being identical nonetheless. Did not reproduce the pattern in the new tests as a result.