feat(api): propagate S3 credentials through load_dataset and add shards property
- load_dataset now creates S3Source with credentials when index has S3DataStore
- Add Dataset.shards property (lazy iterator) following xs/list_xs() convention
- Deprecate Dataset.shard_list in favor of list_shards()
- Update AbstractIndex.data_store to be a @property in the protocol
- Loosen publish_schema signature to accept any type with runtime Packable validation
- Document xs/list_xs() naming convention in CLAUDE.md
- Add test for S3 credential propagation in load_dataset
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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