Add Locale (primitives)

[vhrcgcom]

Description

Implement Locale as a ValueObject in the primitives module.

Spec: BCP 47 language tag (e.g. en-US, cs-CZ)

Implementation checklist

• Create src/primitives/locale.rs
• Implement ValueObject trait
• Add #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
• Export from src/primitives/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	LocaleInput
Output	String	LocaleOutput

Normalization

Trim; normalise the separator: accept both _ and - as separators between the language and region subtags, store with - (BCP 47 canonical form). Lowercase the language subtag, uppercase the region subtag if present (e.g. "en_us" → "en-US").

Validation

Must match BCP 47 at a structural level — for MVP, a two-part rule is sufficient:

• Language subtag: 2–3 ASCII letters (lowercase after normalisation).
• Optional region subtag (after -): exactly 2 ASCII letters (uppercase after normalisation) OR exactly 3 digits.
• Other subtag types (script, variant, extension) are out of scope for MVP.

Extra methods

None beyond the trait.

References

• ROADMAP.md
• CONTRIBUTING.md