Functional: Bond Valuation price/yield calculators (baseline set)

[chrislyons-dev]

Implement price ↔ yield calculators with consistent interfaces.

Instruments / Flows

• Discounted
• Interest at maturity
• Regular coupon
• Odd first/last (all permutations: long/short, first/last)
• Stepped coupon
• Zero coupon

Acceptance

• Shared interface and test vectors
• Day-count integration
• Accrued interest; clean/dirty consistency
• Robust solver with guardrails (e.g., Newton–Raphson)
• Docs with worked examples