feat(api): streamline Dataset API with DictSample default and auto-lens registration
- Add DictSample class for schema-less dataset exploration with dict/attr access
- Update load_dataset to default to DictSample when no type specified
- Auto-register lens from DictSample in @packable decorator for seamless as_type()
- Add comprehensive tests for DictSample creation, serialization, and iteration
- Remove outdated human-review notebook
Closes #338, #339, #340, #341, #342
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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