test: comprehensive v0.8 API coverage (250 tests) - so we can copy these to main later and know what broke.

[lilith]

Exercises the entire v0.8 public API surface:

• All pixel types (RGB, BGR, GRB, RGBA, BGRA, ARGB, ABGR, Gray, GrayAlpha, GrayA)
• All traits (ComponentMap, ColorComponentMap, ComponentSlice, ComponentBytes, AsPixels, FromSlice)
• All inherent methods (new, value, with_alpha, map_alpha, map_gray, rgb, bgr, iter, etc.)
• All From/Into conversions (layout reordering, type widening, gray-to-rgb, array, tuple)
• All operator impls (Add, Sub, Mul, Div, and Assign variants, Sum)
• All formatting (Display, UpperHex, LowerHex)
• Derive impls (Clone, Copy, Debug, Default, Eq, PartialOrd, Hash)
• Feature-gated: as-bytes, bytemuck, serde, checked_fns
• AsRef/AsMut impls
• Memory layout verification

Preliminary discoveries vs main:

1. map_colors() → map_c() — ColorComponentMap renamed its method
2. AsRef<[T]> → AsRef<[T; N]> — RGB/RGBA AsRef returns fixed-size arrays now, not slices
3. Hex formatting — "RGB { #FF0000 }" → "#FF0000" (wrapper removed), BGR hex prints r,g,b semantic order instead of b,g,r memory order,
   u16 components now get 4-digit padding
4. BGRA display — "bgra(r,g,b,a)" → "bgra(b,g,r,a)" (now prints in struct field order)
5. Gray::with_alpha() — removed on main
6. Prelude — exports Pixel/HetPixel/HasAlpha/GainAlpha/ArrayLike instead of ComponentMap/ColorComponentMap