Contaminated feature manifests and leakage cases

Source-derived companion to gp3ml 0.3.0 vignette contaminated-manifest-leakage.Rmd. R code blocks are omitted here; the Python companion script is under examples/contaminated-manifest-leakage.py.

Clean manifest

Deliberate post-outcome contamination

A contaminated manifest must fail or require explicit review before fitting. The example is included to exercise the governance boundary, not to normalize contaminated predictors.