NZ source-only Axiom transport and entitlement smoke

Implement the source-only integration slice of #506: a Pydantic NZ dataset/model adapter through the shared Simulation API; read Microcosm entity-table HDF5 with exact decimal and nullable-boolean decoding; resolve person/family weights through Frame from the sole household weight vector; execute the actual Axiom official-budget module for explicit tax year 2026–27; reject missing inputs and unsupported controls; preserve source files and record input/runtime hashes; provide focused boundary and opt-in real-runtime tests plus an executable example.

This issue does not include a calibrated population, certified country release, fiscal/payment/debt bridges, or completed national Scorecard replications. Those remain tracked in #506 and the NZ country-expansion dependencies.

Start with #506 and the shared Simulation API entry point; trace the NZ adapter boundary, Microcosm entity-table HDF5 inputs, Frame weight resolution, and the Axiom official-budget module for 2026–27. Use the requested focused boundary tests, opt-in real-runtime tests, and executable example as completion checks, including rejection of missing or unsupported inputs and recording source, input, and runtime hashes.