Validate and ingest the refreshed bond-return fields

[Yuning598]

Background

The current bond dataset contains monthly_return and monthly_return_winsorized, but the two fields are presently identical and the latter has not actually been winsorized. The current schema therefore gives a misleading semantic signal and does not yet contain the approved bond excess_return field.

A refreshed dataset is expected to extend coverage through December 2025 and add the excess return defined in #8. This Issue tracks validation and ingestion of that delivery separately from portfolio construction.

Required validation

• Confirm the maximum observation month is December 2025 and identify any coverage gaps.
• Compare monthly_return and monthly_return_winsorized observation by observation.
• Either apply and document a real winsorization rule, rename/remove the misleading field, or prohibit it from analytical use.
• Confirm excess_return is computed as monthly_return minus the aligned one-month U.S. T-Bill return.
• Validate field units, missingness, duplicate bond-month keys, extreme values, and identifier stability.

Acceptance criteria

• The refreshed file has a version identifier, source record, and reproducible schema summary.
• Date coverage through 2025-12 is verified programmatically.
• monthly_return_winsorized no longer claims a transformation that was not performed.
• excess_return is present and matches hand-checked formula calculations.
• Automated ingestion assertions fail on duplicate keys, unexpected units, missing required fields, or silent fallback to raw returns.
• A comparison report documents row counts, coverage, missingness, and differences from the previous dataset.
• Implement one-month T-Bill excess returns for bond portfolios #8 is unblocked only after these checks pass.

Related issues

• Implement amount-outstanding-weighted long-short portfolios for 41 bond characteristics #7 may develop the portfolio mechanics temporarily with explicit monthly_return configuration.
• Implement one-month T-Bill excess returns for bond portfolios #8 owns the final one-month T-Bill excess-return implementation.