### Added
- **Per-channel interpolation**: `Lerp` and `LerpChannel` traits in the `rgb`
module (also re-exported from the prelude). `Lerp` is blanket-implemented for
every `RgbColor` whose channels implement `LerpChannel` — `Rgb888`, `Rgb565`,
`Bgr888`, `Rgbf32`, the ARGB/ABGR types, and custom `Rgb<T>`/`Bgr<T>`.
Interpolation happens in each format's native channel domain (no `Srgb`
round-trip): integer channels round half away from zero and stay exact at
`t = 0.5` (e.g. `0, 255 -> 128`), floating-point channels interpolate without
clamping. Only the red, green, and blue channels are interpolated; alpha and
padding bits are copied from the first operand. Requires neither `std` nor
`libm`.