Model Layer

This module constructs physical model data over the symmetry-aware tensor-network layer. Model builders return ordinary FiniteMps and FiniteMpo owners; they do not define alternate tensor or dispatch types.

Current entry point:

• spin_half_heisenberg.hpp: the U(1) spin-half local space, normalized Néel product MPS, and reduced-boundary open Heisenberg MPO.

The Néel builder produces a rank-one MPS that is canonical in both directions, so it can directly initialize the alternating two-site DMRG run controller in tensor_network/two_site_dmrg.hpp.

Generic spaces and block-sparse operations remain in symmetry/. Finite-chain ownership, environments, splitting, and sweeps remain in tensor_network/. The canonical model contract is documented in `docs/tensor_network/models.md`.