Add VatNumber (finance)

[vhrcgcom]

Description

Implement VatNumber as a ValueObject in the finance module.

Spec: EU VAT number with country-prefix + format validation

Implementation checklist

• Create src/finance/vatnumber.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	VatNumberInput
Output	String	VatNumberOutput

Normalization

Trim, uppercase, strip internal spaces.

Validation

• Must start with a known EU country prefix (2 ASCII letters): AT, BE, BG, CY, CZ, DE, DK, EE, EL, ES, FI, FR, HR, HU, IE, IT, LT, LU, LV, MT, NL, PL, PT, RO, SE, SI, SK, XI.
• After the 2-letter prefix: 2–13 alphanumeric characters.
• For MVP this structural validation is sufficient; per-country format rules (e.g. CZ has 8–10 digits, DE has 9 digits) can be added later.

Extra methods

• country_prefix() -> &str — first 2 characters, e.g. "CZ", "DE".

References

• ROADMAP.md
• CONTRIBUTING.md