Restore Area-Weighted Sampling for SSURGO #3609

Bug Description

Current sampling in extract_soil_gssurgo() uses counts, not true area-weighted aggregation. This can bias soil property estimates, especially for heterogeneous polygons.

Solution

• Use SoilDB package
• Not sure what of the following is handled by that package, but sampling should:
  • Request SSURGO polygon geometries (GML) from WFS for the sampling area.
  • Intersect polygons with sample buffers e.g. using sf or terra.
  • Compute area fractions
  • Aggregate soil properties using these area weights.

References

• Chris' comments and related discussion on PR 3534 [WIP] gSSURGO Enhancements: SOC Integration, enhanced XML Parsing, and Ensemble improvements #3534 (comment)
• Code that previously did area weighting was unintentionally removed in Update gSSURGO queries after apparent API changes #2964