Phase 4: Implement credit and covenant model builder

[DecisionNerd]

Summary

Build a credit and covenant compliance model family.

Part of #40.

Scope

• Support leverage, coverage, liquidity, fixed-charge, and covenant calculations.
• Define covenant input assumptions and thresholds.
• Emit breach and headroom diagnostics.

Acceptance Criteria

• Model output reports covenant status by period.
• Diagnostics identify missing or inconsistent credit assumptions.
• Tests cover representative covenant calculations.