[MID] Add superconducting magnet/cryostat geometry to simulation

[joseesquivel-hub]

Adds the superconducting magnet/cryostat geometry to the MID detector simulation in O2, based on GEANT4 standalone studies by Ian Perez Garcia (ICN-UNAM).

Changes:

• New materials added: Iron (pure Fe), WindingPack (NbTi+Cu+Al composite, Arnaud report v0.2, density 2.96 g/cm3), and Vacuum
• Seven-layer cryostat geometry implemented (inner wall, vacuum gap, winding pack, MLI, support cylinder, vacuum gap, outer wall)
• Two variants supported via MagnetVariant selector: AluminiumWalls (11 cm total) and SteelWalls (10 cm total)
• Magnet positioned at z=-1155 cm, verified with zero geometry overlaps (CheckOverlaps) against the ALICE barrel and MID RPC chambers

Verified with o2-sim: geometry increases from 159 to 167 volumes, 0 illegal overlaps detected.