Add Isbn13 (identifiers) #17

Description

Implement Isbn13 as a ValueObject in the identifiers module.

Spec: ISBN-13 with check digit

Implementation checklist

• Create src/identifiers/isbn13.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

Normalization

Strip hyphens and spaces; keep only digits. Output is 13 bare digits.

Validation

• Exactly 13 digits after stripping.
• Must start with 978 or 979.
• Check digit validated using the EAN-13 algorithm (alternating weights 1 and 3, total mod 10 == 0).

Extra methods

• prefix() -> &str — returns "978" or "979" (first 3 digits).

References

• ROADMAP.md
• CONTRIBUTING.md