Sharded dataset generation

[PatrickRMiles]

Summary

Adds v3 physical sharded dataset generation and loading, with shard files written to match the configured DistConv layout. This lets large samples be generated directly as per-rank physical shards instead of always writing full-volume samples, and should allow us to generate scale 11 data.

Changes

• Generate one set of voxelized fractal indices per logical volume, then render all requested physical shards from that shared data.
• Write v3 dataset metadata containing the physical shard layout and shard-suffixed volume/mask files.
• Load v3 physical shard files directly when the dataset layout matches the requested DistConv layout.
• Preserve v2/full-volume dataset loading for unsharded cache reuse and runtime slicing.
• Canonicalize v3 cache IDs for equivalent shard layouts so reordered dc_num_shards / dc_shard_dims pairs reuse the same dataset cache.

Validation

• Ran sharded dataset equivalence comparisons across problem scales 6, 7, and 8 for layouts 1x1x1, 1x1x2, 1x2x2, and 1x1x4; all 18 pairwise comparisons passed.