updated master parquet script.

pdate configs: remove density splits, add region shift, 90:10 train/val, shared test sets

Export all splits (incl density splits) using streaming

Declare custom splits via README YAML (density-aware OOD splits)

Add density-aware OOD splits and regenerate configs

Make COCO export streaming to avoid OOM on large configs

Replace configs with named international/nation/biome suites

Add COCO export script

Add dataset configs and COCO export tool