Add Slug (identifiers)

[vhrcgcom]

Description

Implement Slug as a ValueObject in the identifiers module.

Spec: lowercase, alphanumeric + hyphens, no leading/trailing hyphens

Implementation checklist

• Create src/identifiers/slug.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	SlugInput
Output	String	SlugOutput

Normalization

Trim surrounding whitespace, convert to lowercase.

Validation

• Must not be empty after trimming.
• May only contain ASCII lowercase letters (a–z), digits (0–9), and hyphens (-).
• Must not start or end with a hyphen.
• Must not contain consecutive hyphens (--).

Extra methods

None beyond the trait — value() returns the normalised slug string.

References

• ROADMAP.md
• CONTRIBUTING.md