Add structural LVA interpolation prototype

[Mining-Geologist]

Summary

Adds the first native structural-LVA replay path without changing the existing Polatory RBF solver or manifold mesher.

Included

• StructuralDomain3 domain specifications
• StructuralInterpolant3 for fitting and blending multiple fixed-anisotropy Polatory interpolants
• StructuralRbfFieldFunction adapter for the existing manifold isosurface generator
• companion Python extension polatory._structural
• automatic Python package export
• regression tests proving a one-domain structural interpolant matches a standard Polatory interpolant

Current scope

This is the replay foundation for the 14-domain Leapfrog A/B experiment. Automatic LVA clustering and exact Leapfrog blend-weight reconstruction are not included yet.

Next steps

• compile and fix any platform-specific issues
• add the exact Leapfrog-domain replay loader
• add the Jupyter comparison notebook using the supplied collar/survey/geology/OBJ data
• validate multi-domain scalar fields and manifold meshes