Add Mexico regulatory framework (NOM-023-STPS-2012)

[Miqueas7]

Same shape as the Colombia request (#4), for Mexico.

Mexican underground mining safety is governed by NOM-023-STPS-2012 (Minas
subterráneas y minas a cielo abierto — Condiciones de seguridad y salud en el
trabajo), with exposure limits in NOM-010-STPS. Those references are the
starting point and part of this issue is confirming them and checking for any
newer revision.

Where the change goes

file	what to add
include/ventpy/normativa.hpp	new RegulatoryStandard enum value + a mexico() config, wired into for_standard()
include/ventpy/limites_gases.hpp	the gas exposure limits for the new standard
bindings/bindings.cpp	expose the enum value to Python
python/ventpy/cli.py	add "mexico" to the _NORMAS map
tests	one case per threshold, citing the article it comes from

What is actually needed

Same as Colombia: the sourced table, with the article cited for every number —
air per person, air per brake horsepower for diesel equipment, velocity limits by
working type, gas exposure limits and O₂ minimum, re-entry time after blasting.

A note specific to Mexico: exposure limits may live in a different NOM from the
ventilation requirements themselves. If that is the case, both should be cited
separately rather than merged into one source, so the traceability matrix stays
honest about where each number comes from.

Comment here if you work under this framework — the sourced table is the part that
matters, and the C++ side can be handled from there.