nat.fastcore - Re-implementation of nat core functions in Rust

A re-implementation of core 'nat' (Neuroanatomy Toolbox) functions in Rust for speed. Provides fast skeleton/tree (DAG) operations (geodesic distances, Strahler order, segment generation, twig pruning, synapse flow centrality, node classification, skeleton healing), mesh connected components, and NBLAST/synNBLAST neuron similarity scoring, exposed to R via extendr.