[Mechanics_OG] Serialization of mechanical state

[joseestragues]

Summary
Add CSV serializer for mechanical agent state.

Steps

1. Header mechanics/physicell/include/vtk_mechanics_serializer.h with class vtk_mechanics_serializer.
2. Inherit from serializer and implement the methods
3. Implementation in mechanics/physicell/src/vtk_mechanics_serializer.cpp.
4. Look at MicroMechanics vtk-output branch

Acceptance Criteria

1. File appears with correct columns for a small run.

Verification
Run environment with serialization enabled; inspect file.

Dependencies
Agent container.

Follow-ups
Environment tests verifying file existence.