CMYK→sRGB conversion produces different results than skcms/libjxl

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## Summary

When using moxcms to convert CMYK images to sRGB using an embedded ICC profile, the results differ from libjxl's reference output (which uses skcms).

## Context

We're implementing a JPEG XL decoder in Rust (jxl-rs) and using moxcms for color management. For CMYK images with embedded ICC profiles, we need to convert CMYK→sRGB using the profile's lookup tables.

## Test Case

**Source file**: `cmyk_layers.jxl` from the [codec-corpus](https://github.com/imazen/codec-corpus) JXL conformance test suite.

**File details**:
- Image size: 512x512
- Color space: CMYK (stored as RGB + Black extra channel)
- Embedded ICC profile: ~557KB CMYK profile
- 4 frames (multi-layer composition)

## Current Results

| Metric | moxcms (Perceptual) | libjxl/skcms |
|--------|---------------------|--------------|
| max_error | 60 | 0 (reference) |
| error_count | 251,108 (24%) | 0 |

Example pixel at (0,0):
- Reference (djxl/skcms): `[252, 254, 255, 255]` (near white)
- moxcms output: `[255, 255, 255, 255]` (pure white)

Input CMYK values (already inverted from JXL convention):
- C=0.0, M=0.0, Y=0.0, K=0.0 (no ink = white paper)

## Code

```rust
// Creating the transform
let src_profile = moxcms::ColorProfile::new_from_slice(&cmyk_icc_data)?;
let dst_profile = moxcms::ColorProfile::new_srgb();
let options = moxcms::TransformOptions::default(); // Perceptual intent

let transform = src_profile.create_transform_f32(
moxcms::Layout::Rgba, // CMYK 4 channels
&dst_profile,
moxcms::Layout::Rgb, // sRGB 3 channels
options
)?;

// Applying the transform
let cmyk_input: Vec = vec![0.0, 0.0, 0.0, 0.0]; // no ink = white
let mut rgb_output: Vec = vec![0.0; 3];
transform.transform(&cmyk_input, &mut rgb_output)?;
// Output: [0.9999, 1.0, 1.0] ≈ [255, 255, 255]
// Expected: [0.988, 0.996, 1.0] ≈ [252, 254, 255]
```

## Notes

1. **Value convention**: JXL stores CMYK as 1=no ink, 0=max ink. We invert these before passing to moxcms (so moxcms receives standard ICC convention: 0=no ink, 1=max ink).

2. **Rendering intent**: We tested Perceptual, RelativeColorimetric, and AbsoluteColorimetric. Perceptual gives the best results (lowest error_count), but all produce different results than skcms.

3. **The difference is small but consistent**: Most errors are 1-3 units out of 255, but some reach 60 units. The pattern suggests different LUT interpolation or precision.

## Questions

1. Are there known differences between moxcms and skcms in CMYK handling?
2. Is there a specific configuration that would make moxcms more closely match skcms output?
3. Should moxcms read the rendering intent from the ICC profile header rather than using TransformOptions?

## To Reproduce

1. Clone [codec-corpus](https://github.com/imazen/codec-corpus)
2. Run `./jxl/generate_references.sh` with djxl to generate reference outputs
3. Use moxcms to transform the CMYK values from `cmyk_layers.jxl` through its embedded ICC profile
4. Compare with the reference output

The embedded ICC profile can be extracted from the JXL file - it's stored after the file header. The JXL file is available at: https://github.com/imazen/codec-corpus/blob/main/jxl/conformance/cmyk_layers.jxl

## Environment

- moxcms version: (using latest from crates.io via Cargo)
- Rust: stable
- Platform: Linux x86_64

Start by reproducing the mismatch with cmyk_layers.jxl from the codec-corpus and the embedded ICC profile, using create_transform_f32 and transform. Run jxl/generate_references.sh with djxl, then compare moxcms output against the skcms reference across rendering intents. Done means identifying the cause or supported configuration and documenting a reproducible comparison or regression test.

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