This adds implementations of `std::iter::Sum` for `Pixels`, allowing the
use of `.sum()` on iterators of `Pixels` values.

### Changes
- Implement `Sum<Pixels>` for `Pixels` (owned values)
- Implement `Sum<&Pixels>` for `Pixels` (references)

This enables ergonomic patterns like:
```rust
let total: Pixels = pixel_values.iter().sum();
```