Add optional `serde` dependency with `Serialize` and `Deserialize` implementations on `Component` and all 22 color spaces. Alpha is omitted from serialization when opaque (1.0) and defaults to 1.0 on deserialization when absent. The feature is included in `full`. - Custom `Serialize`/`Deserialize` impls on `Component` to serialize as a bare `f64` - Custom impls on `Rgb` and cylindrical/subtractive spaces to handle generic type parameters and non-serializable fields - Derive-based impls on CIE, perceptual, and physiological spaces - Integration tests covering roundtrip, alpha handling, and field names for `Xyz`, `Rgb`, `Lms`, `Lab`, `Oklch`, `Hsl`, and `Cmyk`