Add HexColor (primitives) #73

Description

Implement HexColor as a ValueObject in the primitives module.

Spec: #RRGGBB or #RGB, normalised to uppercase

Implementation checklist

• Create src/primitives/hexcolor.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

Trim; uppercase; expand 3-digit shorthand to 6-digit form (#F0A → #FF00AA by doubling each hex digit).

Validation

• Must start with #.
• Remaining characters must be exactly 3 or 6 hexadecimal digits (0–9, A–F after uppercasing).

Extra methods

• r() -> u8 — red channel value (0–255).
• g() -> u8 — green channel value (0–255).
• b() -> u8 — blue channel value (0–255).

References

• ROADMAP.md
• CONTRIBUTING.md