Add Ean8 (identifiers)

[vhrcgcom]

Description

Implement Ean8 as a ValueObject in the identifiers module.

Spec: EAN-8 barcode with checksum validation

Implementation checklist

• Create src/identifiers/ean8.rs
• Implement ValueObject trait
• Add #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
• Export from src/identifiers/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	Ean8Input
Output	String	Ean8Output

Normalization

Strip all spaces and hyphens, keep only digits.

Validation

• Exactly 8 digits after stripping.
• Last digit is a valid EAN-8 check digit (same algorithm as EAN-13 but applied to 8 digits).

Extra methods

• check_digit() -> u8 — returns the check digit (last digit).

References

• ROADMAP.md
• CONTRIBUTING.md