Add KL bar plots and sample_frac to load_predicted_local; ignore root parquet scratch files

Adds plot_kl_bars/plot_kl_bars_pl (numpy/polars variants) for ranking which
target dimension or pdg/material stratum drives KL regressions, with the
same kl*n group capping as plot_marginals. Switches existing histogram
plots to step-type/log-scale. Adds sample_frac to load_predicted_local for
subsampling large predict parquets.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -11,3 +11,6 @@ wheels/
 
 # Training results
 checkpoints/
+
+# Predicted/scratch parquet files dropped in the repo root
+/*.parquet