Use vendored data in Hansen-Singleton lectures

[nyxst4ck]

Summary

• read both Hansen-Singleton datasets from their vendored _static/lecture_specific paths
• update the surrounding prose to describe the checked-out data source
• remove runtime dependence on raw GitHub URLs

Closes #958

Validation

• pandas smoke checks: 1982 data (239, 2) and 1983 data (239, 5), with expected columns and DatetimeIndex
• executed and built hansen_singleton_1982 successfully (all cells, HTML)
• executed and built hansen_singleton_1983 successfully (all cells, HTML)
• git diff --check passes