htna: Heterogeneous Transition Network Analysis

Implements the Heterogeneous Transition Network Analysis (HTNA) method described by López-Pernas et al. (2026) . The method is an extension of transition network analysis (TNA) where actions or events belong to two or more distinct actor types (e.g. Human and AI),preserving the actor type partition on the resulting network. Provides a thin, focused API on top of the 'Nestimate' estimation engine and the 'cograph' rendering engine, so downstream bootstrap, permutation, reliability, centrality, and plotting functions treat each actor's codes as a distinct node group.