Add sparse ICC v4 mAB/mBA LUT support for ICC conversion

[JimBobSquarePants]

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Description

Fixes #3064

Adds support for valid sparse ICC v4 mAB/mBA pipelines, including transforms such as A -> CLUT -> Matrix.
Updates LUT tag validation to derive channel counts from stage continuity, updates LutABCalculator to execute only the stages present in ICC order, and adds documentation for the new behavior.

This fixes decode failures for embedded ICC profiles like Display P3 PQ profiles.

[Copilot]

Pull request overview

Adds support for sparse ICC v4 mAB/mBA LUT pipelines (eg. A -> CLUT -> Matrix) during ICC-based color conversion to fix decode failures for certain embedded profiles (Issue #3064 / Display P3 PQ).

Changes:

• Update LutAToB/LutBToA tag validation to derive channel counts from stage continuity (supports sparse stage combinations).
• Update LutABCalculator to execute only the stages present, in ICC specification order for mAB/mBA.
• Add a regression test image + reference output for Issue #3064.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/Images/Input/Jpg/icc-profiles/issue-3064.jpg	Adds regression JPEG containing the problematic embedded ICC profile (LFS).
tests/Images/External/ReferenceOutput/JpegDecoderTests/Decode_RGB_ICC_Jpeg_Issue3064_Rgba32_issue-3064.png	Adds expected decode output for regression comparison (LFS).
tests/ImageSharp.Tests/TestImages.cs	Registers the new ICC regression test image path.
tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.cs	Adds a decoder test that exercises ICC conversion for Issue #3064.
src/ImageSharp/Metadata/Profiles/ICC/TagDataEntries/IccLutBToATagDataEntry.cs	Reworks LUT stage validation to support sparse mBA pipelines and derive channel counts.
src/ImageSharp/Metadata/Profiles/ICC/TagDataEntries/IccLutAToBTagDataEntry.cs	Reworks LUT stage validation to support sparse mAB pipelines and derive channel counts.
src/ImageSharp/Metadata/Profiles/ICC/IccProfile.cs	Minor refactor to use range-based slicing when restoring hash inputs.
src/ImageSharp/Metadata/Profiles/ICC/DataReader/IccDataReader.TagDataEntry.cs	Refactors tag entry dispatch to a switch expression; makes a local helper static.
src/ImageSharp/ColorProfiles/Icc/IccConverterbase.Conversions.cs	Improves Invalid ICC entry exception message by including the tag.
src/ImageSharp/ColorProfiles/Icc/Calculators/LutABCalculator.cs	Applies only present LUT stages in ICC order; removes legacy “fixed combinations” logic.
src/ImageSharp/ColorProfiles/Icc/Calculators/LutABCalculator.CalculationType.cs	Simplifies calculation type enum to direction-only.

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