CRUX M7: Data depth — 11 missing stories

[noahgift]

Parent: #917

Scope

HuggingFace datasets parity — streaming, sharding, parquet, arrow interop, WebDataset.

Contracts in scope

• contracts/crux-H-{03,06,12,13,14,16,17,18,19,20,21}-v1.yaml

Exit criteria

• 11 contracts promoted missing → supported
• New apr dataset surface OR integration into existing apr pretrain --dataset
• ShardBatchIter handles all 11 format/streaming patterns

Dependencies

• Existing: ShardBatchIter (reads .bin u32 LE)
• Blocks on: pretokenize-bin-v1 contract (task APR-POKA-001: Poka-yoke validation for AprWriter #123 shipped)