Add CardExpiryDate (finance)

[vhrcgcom]

Description

Implement CardExpiryDate as a ValueObject in the finance module.

Spec: MM/YY; rejected if in the past at construction time

Implementation checklist

• Create src/finance/cardexpirydate.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 — accepts "MM/YY" or "MM/YYYY"	CardExpiryDateInput
Output	String — normalised as "MM/YY"	CardExpiryDateOutput

Normalization

Parse month and year; normalise to 2-digit year for storage and output.

Validation

• Month must be 01–12.
• The card must not have already expired: the expiry month/year must be >= the current month/year at construction time (a card is valid for the entire month of its expiry).

Extra methods

• month() -> u8 — month as a number, 1–12.
• year() -> u16 — 4-digit year, e.g. 2027.

References

• ROADMAP.md
• CONTRIBUTING.md