Fix grayscale ICC profile conversion - #3199
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Description
Fixes #3197.
Converting a grayscale JPEG with an embedded ICC profile could wash out its tones because the gray TRC result was not expanded into the profile connection space. Construct neutral D50 XYZ or Lab values from the curve result, and use XYZ luminance or Lab lightness for the inverse conversion. Evaluate the single curve directly and precompute the encoded D50 white point.
Add XYZ/Lab calculator tests and a JPEG regression using the reporter's image with a full-image reference generated by LittleCMS 2.19 using the embedded source profile and CompactSrgbV4Profile target.
Validation: the full Release suite passed on .NET 8 and .NET 10, each with 47,814 passed, 11 skipped, and no failures. After the final test-only change to reuse KnownIlluminants.D50Icc, all 12 affected tests passed again on both runtimes.
Overlap check: #3023 touches the same ICC files for nullable annotations on main. This fix targets release/4.1.x; no duplicate fix was found.