Refactor SimpolPhaseTransfer in musica.mechanism_configuration to use composition instead of inheritance from _SimpolPhaseTransfer.

Acceptance Criteria

• Class no longer inherits from _SimpolPhaseTransfer.
• Holds internal C++ instance in self._instance.
• Attributes (name, gas_phase, gas_phase_species, aerosol_phase, aerosol_phase_species, B, other_properties) delegate to self._instance.
• gas_phase_species and aerosol_phase_species accept Python Species or (float, Species), converting to/from C++ _ReactionComponent.
• Validate B as list of length 4.
• serialize() converted to instance method using only Python-visible data.
• Public interface exposes only Python-native types without direct C++ inheritance.

Ideas

• Follow tuvx.py wrapper pattern.
• Use @property decorators for attribute delegation and conversion.
• Refactor serialize() as instance method.
• Use helper functions for Python-C++ conversion of reaction components.
• Do the work similar to Refactor CondensedPhaseArrhenius to use composition instead of inheritance #461 Refactor Arrhenius class to use composition instead of inheritance #462, Refactor Troe class from inheritance to composition pattern #466, Refactor Branched class to use composition instead of inheritance #467, Refactor AqueousEquilibrium to use composition instead of inheritance #475, Refactor Tunneling class to use composition instead of inheritance #476

Tasks

• Remove _SimpolPhaseTransfer inheritance.
• Initialize self._instance inside __init__.
• Forward attribute access/mutation to self._instance with properties.
• Implement validation for B on set.
• Refactor serialize() to instance method with Python attributes.
• Update code that calls serialize() accordingly.