Fix CMYK blending order: blend in CMYK space then convert to RGB

[hjanuschka]

Summary

Fix multi-layer CMYK image blending by deferring CMS conversion until after blending operations complete.

Problem: For CMYK images with alpha blending across multiple layers, blending in RGB space (after CMS conversion) produces incorrect results. The correct approach is to blend in CMYK space first, then convert to RGB.

Solution: Defer CMS conversion for CMYK images when:

1. The frame needs blending, OR
2. The frame can be referenced by later frames (save_before_ct=false)

This ensures reference frames are saved in CMYK space so subsequent frames blend correctly before final CMS conversion.

Changes

• jxl/src/frame/render.rs: Restructured CMS stage application to support deferred execution for CMYK blending

Credit

Based on the fix from @lilith in lilith/jxl-rs.

[github-actions]

Benchmark @ 6170405

MULTI-FILE BENCHMARK RESULTS (4 files)
  CPU architecture: x86_64
  WARNING: System appears noisy: high system load (2.73). Results may be unreliable.
Statistics:
  Confidence:               99.0%
  Max relative error:        3.0%

Comparing: a5ae7dc1 (Base) vs f0ec2aa2 (PR)

File	Base (MP/s)	PR (MP/s)	Δ%
bike.jxl	22.614	22.889	+1.22% ±3.0%
green_queen_modular_e3.jxl	7.878	7.896	+0.23% ±1.0%
green_queen_vardct_e3.jxl	21.108	20.948	-0.76% ±2.0%
sunset_logo.jxl	2.228	2.229	+0.04% ±0.3%

[jonsneyers]

For multi-layer images, not only in the case of CMYK, the frame blending has to be done in the image space, not in the final decoded image space. This also makes a difference in the RGB case, e.g. if the image space is linear Rec2020 (i.e. that's what the imageheader says), and we're decoding to sRGB, then frames have to be kept in or converted to linear Rec2020 for frame blending and only after blending the result should be converted to the final decode space.

Are we doing this correctly?
It's a subtle point but it does make a difference.