Make color generic over premul types

Per my comment in https://github.com/linebender/color/pull/190#issuecomment-3196190653.

Sketch:

// similar to colorspacetag
enum AlphaType {
    Alpha,
    PremultipliedAlpha,
}

struct Alpha;
struct PremultipliedAlpha;

// for const generic
// alpha trait is like colorspace trait
impl AlphaTrait for Alpha {
    const ALPHA = AlphaType::Alpha;
}

impl AlphaTrait for AlphaPremultiplied {
    const ALPHA = AlphaType::AlphaPremultiplied;
}

pub struct AlphaColor<CS, A> {
    /// The components, which may be manipulated directly.
    ///
    /// The interpretation of the first three components depends on the color
    /// space. The fourth component is separate alpha.
    pub components: [f32; 4],
    /// The color space.
    pub cs: PhantomData<CS>,
    pub alpha: PhantomData<A>,
}

pub struct DynamicColor {
    pub alpha: AlpahType,
    /// The color space.
    pub cs: ColorSpaceTag,
    /// The state of this color, tracking whether it has missing components and how it was
    /// constructed. See the documentation of [`Flags`] for more information.
    pub flags: Flags,
    /// The components.
    ///
    /// The first three components are interpreted according to the
    /// color space tag. The fourth component is alpha, interpreted
    /// as separate alpha.
    pub components: [f32; 4],
}

PremulColor would be AlphaColor<_, PremiltipliedAlpha> so we can share more impls and DynamicColor is now also dynamic over AlphaType.

Motivation:

• This would also allow us to deduplicated (un)premultiplied interpolation/gradient stuff.
• https://github.com/linebender/color/pull/190#issuecomment-3196190653:

  One color variant says "I have this kind of [alpha]" (you need to know that premul is an abbreviation for a word and that this word pertains to alpha).
  The other says "I have alpha".

This would be break change.

Start with the discussion in pull request 190 and the AlphaColor, PremulColor, and DynamicColor entry points described here. Compare their current color-space and alpha representations, then define the breaking generic API and shared interpolation behavior; done means the proposed alpha typing is consistent across static and dynamic colors.