Add Weight (measurement)

[vhrcgcom]

Description

Implement Weight as a ValueObject in the measurement module.

Spec: non-negative f64 with unit (kg, g, lb, oz)

Implementation checklist

• Create src/measurement/weight.rs
• Implement ValueObject trait
• Add #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
• Export from src/measurement/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

Weight is a composite value object that carries both a numeric value and a unit.

	Type
Input	WeightInput { value: f64, unit: WeightUnit }
Output	String — e.g. "70.0 kg", "154.32 lb"

Define a unit enum in the same file:

WeightUnit { Kg, G, Lb, Oz }

WeightUnit must derive Debug, Clone, Copy, PartialEq.

Validation

• value must be >= 0.0 and finite.

Extra methods

• amount() -> f64
• unit() -> WeightUnit
• to_kilograms() -> f64 — convert to the SI base unit at call time.

References

• ROADMAP.md
• CONTRIBUTING.md