Evaluate whether solvent_floor and min_halflife fields belong in the public API

Those two fields were added in aerosol.hpp to support the musica tutorials.

std::optional<double> solvent_floor_;
std::optional<double> min_halflife_;

At the moment, these two fields are not wired into the configuration system. There are no corresponding YAML keys, the configuration parser does not populate them, and validate.cpp does not include any validation logic for them.

Evaluate whether these fields belong in the public API. They may exist solely to support the current solver's numerical stability and may not make sense as user-configurable options.

Notes: Matt mentioned that they are used for condensed-phase reactions that depend on solution concentrations, preventing concentrations from becoming infinite if the solvent evaporates completely.

Start by reading the field declarations in aerosol.hpp, then trace the configuration parser and the validation path in validate.cpp. Determine whether solvent_floor_ and min_halflife_ are intended as user-configurable API fields or internal solver state; done means recording a clear decision and its implications for YAML configuration and validation.