Nested tuning without information leakage

Candidate parameters must be compared on fixed inner partitions. When an outer design is requested, ps_tune_interpolation() sees each outer holdout only after selecting parameters inside that outer training set. Every failed candidate remains in the manifest. Without outer validation, scores are tuning performance and should not be presented as unbiased final performance.

tuned <- ps_tune_interpolation(
  points, "IDW", list(idw_power=c(1, 1.5, 2), idw_nmax=c(8, 15)),
  inner_design="spatial_block", outer_design="kfold", seed=31
)