This PR refactors the SimpolPhaseTransfer class in musica.mechanism_configuration to use composition instead of inheritance from _SimpolPhaseTransfer, following the established pattern used in the Arrhenius class.

Changes Made

Core Refactoring

• Removed inheritance: SimpolPhaseTransfer no longer inherits from _SimpolPhaseTransfer
• Added composition: Uses self._instance = _SimpolPhaseTransfer() to hold the internal C++ instance
• Property delegation: All attributes now use @property decorators to delegate to self._instance

Attribute Handling

• Type conversion: gas_phase_species and aerosol_phase_species properties convert between Python Species/(float, Species) tuples and C++ _ReactionComponent objects
• Phase handling: gas_phase and aerosol_phase accept both Phase objects and strings, converting to string names for the C++ instance
• Validation: B parameter is validated as a list of exactly 4 elements on assignment

Serialization Updates

• Instance method: Converted serialize() from static method to instance method using only Python-visible data
• Static compatibility: Added serialize_static() method for backward compatibility with C++ _SimpolPhaseTransfer objects
• Mechanism configuration: Updated mechanism_configuration.py to handle both C++ and Python instances separately

Public Interface

• Type property: Added type property returning ReactionType.SimpolPhaseTransfer
• API preserved: Public interface remains unchanged - existing code continues to work
• Python-native types: Public interface exposes only Python-native types without direct C++ inheritance

Example Usage

import musica.mechanism_configuration as mc

# Create species and phases
ethanol = mc.Species(name="ethanol")
ethanol_aq = mc.Species(name="ethanol_aq")
gas = mc.Phase(name="gas")
aerosol = mc.Phase(name="aerosol")

# Usage remains the same
simpol = mc.SimpolPhaseTransfer(
    name="my simpol",
    gas_phase=gas,
    gas_phase_species=ethanol,
    aerosol_phase=aerosol,
    aerosol_phase_species=ethanol_aq,
    B=[-1.97e03, 2.91e00, 1.96e-03, -4.96e-01],
    other_properties={"test": "value"}
)

# Properties work transparently
assert simpol.name == "my simpol"
assert simpol.gas_phase == "gas"
assert len(simpol.B) == 4

# Serialization works as instance method
result = simpol.serialize()

Validation

All existing tests continue to pass as the public API remains unchanged. The refactoring follows the same pattern established in similar classes like Arrhenius.

Fixes #452.

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