sharp() constructor for raw pixel input has no colorspace parameter — blocks CMYK raw roundtrip

Nobody has claimed this yet.

## Feature request

When constructing a Sharp instance from a raw pixel buffer, there is no way to declare the colorspace of those bytes. The `raw` input option only accepts `{ width, height, channels, depth? }`. Four-channel raw input is always interpreted as RGBA, making it impossible to feed modified CMYK raw bytes back into Sharp for re-encoding as a CMYK JPEG.

### What are you trying to achieve?

This blocks any workflow that needs to: decode a CMYK image → manipulate individual channel values in JS → re-encode as CMYK JPEG, without a lossy sRGB round-trip.

### When you searched for similar feature requests, what did you find that might be related?

Issue #3620 fixed the `pipelineColorspace` CMYK roundtrip in v0.32.1, which is related but does not address raw input colorspace declaration.
Also saw https://github.com/lovell/sharp/issues/3789

### What would you expect the API to look like?

```js
sharp(rawCmykBuffer, {
raw: {
width: 400,
height: 300,
channels: 4,
colorspace: 'cmyk' // <-- missing option
}
})
.toColorspace('cmyk')
.jpeg({ quality: 95 })
.toBuffer()
```

Alternatively, a `pipelineColorspace` hint on the constructor that applies to raw input would achieve the same result.

### What alternatives have you considered?

converting to RGB, but want to avoid conversion potential loss. Output image is embedded in .pdf for CMYK printing.

### Please provide sample image(s) that help explain this feature

Any image.

Start at the sharp() constructor's raw input option and trace how raw four-channel buffers are interpreted before toColorspace('cmyk') and jpeg() encoding. Review the related behavior described in issues #3620 and #3789. Done means a raw input can declare CMYK colorspace and preserve the requested CMYK roundtrip without requiring RGB conversion.

Written by the indexing model from the issue text.