Add New Zealand Business Number (NZBN) validation

[ryanduguid]

Summary

Add stdnum.nz.nzbn for New Zealand Business Numbers, with compact(), validate() and is_valid(). Validation reuses EAN normalisation and the GS1 checksum, checking digits, length 13 and the documented 942 prefix in that order.

The official API documentation specifies the length and check digit; Companies Office guidance specifies 942. The module accepts that prefix without requiring the narrower 9429 seen in common examples. It does not confirm NZBN assignment or perform online lookups.

Closes #105.

Evidence

• Before: get_cc_module('nz', 'nzbn') returns None, and importing the module raises ImportError on master 006192e.

• After: 415 tests pass with 100% branch coverage on Windows CPython 3.8 to 3.14 and Linux CPython 3.14; nine conditional tests skip on each run. The new module covers all 20 statements and six branches.

• Linux tox -e py314,flake8,mypy,docs,headers passes. Mypy checks the declared Python 3.9 to 3.14 targets. Windows flake8 also passes with UTF-8 enabled.

• Doctests include 20 public corporate NZBNs from the Peppol Directory, a separate official API example, normalisation and failure cases. Sources and collection date are recorded; synthetic cases make no assignment claim.

• Source archive and wheel contain the exact module bytes. Fresh installed-package checks confirm country-module discovery and the existing NZ VAT alias, including a fresh wheel install on Python 3.8.

• Upstream: Test workflow passes CPython 3.8 to 3.14, PyPy 3.9, lint, mypy, docs, headers and CodeQL. All 14 check runs on the published head report success.

Merge danger

Door: two-way. Blast radius: NZBN callers and module catalogues.

This adds a module and its doctests. Existing validators, aliases, dependencies and release documentation are unchanged. Prefix enforcement follows the current published 942 description; it does not establish registration status.

Unverified

macOS was not tested. Local checks covered Windows CPython 3.8 to 3.14 and Linux CPython 3.14; upstream CI covers the remaining declared Linux matrix, including PyPy 3.9. Live registry assignment is outside this module's scope.

The unchanged Windows checks have platform issues: default text decoding fails in flake8 and the header script. With UTF-8 enabled, flake8 passes; the header script reports 15 identification failures in unchanged update scripts and flags neither added file. Linux's complete header check passes.