feat: opt-in local-radius RBF for InterpolatedStructure dip interpolation

[lachlangrose]

Summary

Follow-up to a review of the thickness calculators. InterpolatedStructure interpolates dip with scipy.interpolate.Rbf, which fits one smooth surface to every structure measurement across the whole map at once. Near a fold hinge, fault, or unconformity, true dip can change quickly over a short distance, and a single global surface blurs that change into neighbouring, structurally unrelated units — feeding a wrong dip value into the thickness = L * sin(dip) formula.

This adds an opt-in local-radius mode:

• DipDipDirectionInterpolator gains a neighbors: Optional[int] constructor argument. When interpolate() is called with scipy.interpolate.RBFInterpolator (new — previously only Rbf/LinearNDInterpolator were supported), each grid point's fit uses only its neighbors nearest structure measurements via RBFInterpolator's built-in neighbors parameter, rather than every point on the map.
• InterpolatedStructure gains local_interpolation_neighbors: Optional[int] = None. When set, it builds the dip interpolator in local mode; when left as None (the default), behaviour is byte-for-byte the same whole-map Rbf fit as before.

This is purely additive — nothing changes for existing callers unless they explicitly opt in.

Test plan

• pytest tests/thickness tests/project/test_thickness_calculations.py — passes unchanged with the new parameter left at its default
• Manual run of InterpolatedStructure on the Hamersley test fixtures with local_interpolation_neighbors unset, 5, and 20 — all three run cleanly with no NaNs, and thickness estimates shift sensibly as the neighbourhood shrinks

[lachlangrose]

Closing — map2loop now lives in packages/map2loop of Loop3D/LoopStructural, and this repository is being archived. Ported to Loop3D/LoopStructural#329