Add dataset visualization train_val_test_class_distribution_merged.png

Add dataset visualization site_class_pixel_counts_heatmap_merged.png

Add dataset visualization site_distribution_osm.png

Clarify Croissant availability and requirements for private datasets

Add dataset card with data details, split, class schema, and Croissant notes

Flatten dataset layout: data/data/* -> {train,val,test}/*

Add GoldMiningMDD data folder from previous model repo

initial commit