Add Iban (finance)

[vhrcgcom]

Description

Implement Iban as a ValueObject in the finance module.

Spec: IBAN with mod-97 checksum

Implementation checklist

• Create src/finance/iban.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	IbanInput
Output	String	IbanOutput

Normalization

Strip all spaces, convert to uppercase. Store internally without spaces.

Validation

• Length 15–34 characters after stripping.
• First 2 characters must be ASCII letters (country code).
• Characters 3–4 must be digits (check digits).
• All remaining characters must be alphanumeric.
• Validate using the mod-97 algorithm: move the first 4 characters to the end, replace each letter with its numeric value (A=10, B=11, …, Z=35), interpret the result as a big integer, check that it mod 97 == 1.

Extra methods

• country_code() -> &str — first 2 characters, e.g. "CZ".
• check_digits() -> &str — characters 3–4, e.g. "55".
• bban() -> &str — Basic Bank Account Number, characters 5 onwards.

References

• ROADMAP.md
• CONTRIBUTING.md