Ice rules: On each tetrahedron vertex (here: square plaquette analogue), spins must satisfy
2-in 2-out. This mimics hydrogen positions in water ice (Pauling 1935 entropy: S=Nk·ln(3/2)/2).
Violations create emergent magnetic monopoles — defects that behave like free Dirac monopoles,
connected by Dirac strings of reversed spins. At low T, the system freezes into a degenerate manifold.