Add ExchangeRate (finance)

[vhrcgcom]

Description

Implement ExchangeRate as a ValueObject in the finance module.

Spec: positive Decimal, from/to currency pair

Implementation checklist

• Create src/finance/exchangerate.rs
• Implement ValueObject trait
• Add #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
• Export from src/finance/mod.rs and prelude
• Unit tests: valid input · empty input · invalid format · normalisation
• Doc comment with # Example block
• Update status in ROADMAP.md from ⬜ to ✅

Implementation detail

Input / Output

ExchangeRate is a composite value object.

	Type
Input	ExchangeRateInput { from: CurrencyCode, to: CurrencyCode, rate: rust_decimal::Decimal }
Output	String — e.g. "EUR/USD 1.0850"

Validation

• rate must be strictly positive (> 0).
• from and to must differ — an exchange rate from a currency to itself is meaningless.
• Both from and to are already validated CurrencyCode instances.

Extra methods

• from() -> &CurrencyCode
• to() -> &CurrencyCode
• rate() -> &rust_decimal::Decimal

References

• ROADMAP.md
• CONTRIBUTING.md