Add NonNegativeInt (primitives) #69

Description

Implement NonNegativeInt as a ValueObject in the primitives module.

Spec: i64 >= 0

Implementation checklist

• Create src/primitives/nonnegativeint.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

Normalization

None.

Validation

• Must be >= 0. Negative values are rejected.

Extra methods

None beyond the trait.

References

• ROADMAP.md
• CONTRIBUTING.md