docs: add architecture overview and expand tutorials with design context
- Add reference/architecture.qmd with system design overview
- Expand quickstart, local-workflow, atmosphere, and promotion tutorials
- Expand index.qmd with architectural narrative
- Convert TODO comments to design notes in dataset.py
- Replace no-op `_ = None` with `pass` in _stub_manager.py
- Add tests for empty tar file iteration edge cases
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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