Add CurrencyCode (finance)

[vhrcgcom]

Description

Implement CurrencyCode as a ValueObject in the finance module.

Spec: ISO 4217 alpha-3 (EUR, USD, CZK…)

Implementation checklist

• Create src/finance/currencycode.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

	Type	Alias
Input	String	CurrencyCodeInput
Output	String	CurrencyCodeOutput

Normalization

Trim, convert to uppercase.

Validation

• Exactly 3 ASCII letters.
• Must be a known ISO 4217 alphabetic code (embed a static lookup table of all ~170 active currency codes, similar to the calling_code map in PhoneNumber).

Extra methods

None beyond the trait.

References

• ROADMAP.md
• CONTRIBUTING.md