Add Issn (identifiers)

[vhrcgcom]

Description

Implement Issn as a ValueObject in the identifiers module.

Spec: ISSN with check digit

Implementation checklist

• Create src/identifiers/issn.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	IssnInput
Output	String	IssnOutput

Normalization

Strip spaces and hyphens; keep only digits and a possible trailing X. Format the output as XXXX-XXXX (with the hyphen reinserted after the 4th character).

Validation

• Exactly 8 characters after stripping (7 digits + check character which is 0–9 or X).
• Check character validated using ISSN weighted sum: multiply each of the first 7 digits by weights 8 down to 2, sum them, check character makes the total mod 11 == 0 (X represents 10).

Extra methods

None beyond the trait. value() returns the formatted XXXX-XXXX string.

References

• ROADMAP.md
• CONTRIBUTING.md