5GB: update README for sharded parquet 50M

Viewer fix: keep only default parquet config (json still in repo as files, not viewer config) + Problem Types above Quickstart

Fix viewer: json array -> jsonl lines + move Problem Types above Quickstart

Docs: clarify parquet/json dual folders and working load_dataset examples

Reorganize into parquet/ and json/ folders + remove em dashes - verified dual configs

Remove license comparison section

Initial release: 150K verified arithmetic reasoning (v4) with tiered difficulty