[ndavat]

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Description

Fixes #1129. Implements the API direction agreed in discussion #1358.

New public namespace SixLabors.ImageSharp.Comparison:

• ImageComparer - abstract comparer with Exact, Tolerant(...) and TolerantPercentage(...) factories. Non-generic entry points: CompareImagesOrFrames(Image, Image), CompareImagesOrFrames(int, ImageFrame, ImageFrame), CompareImages(Image, Image) and VerifySimilarity(Image, Image).
• TolerantImageComparer - configurable thresholds with the same semantics as the internal test comparers: manhattan metric in sixteen-bit units, same normalization, NaN-safe comparisons.
• ImageSimilarityReport - non-generic report: frame index, compared frames, TotalNormalizedDifference, DifferencePercentageString, IsEmpty, and CreateDifferenceImage().
• Exceptions: ImagesSimilarityException, ImageDimensionsMismatchException, ImageFramesMismatchException, ImageDifferenceIsOverThresholdException.

Design notes (from discussion #1358):

• Pixel-agnostic: entry points take non-generic Image / ImageFrame. A pair of nested IImageFrameVisitor implementations double-dispatches into the pixel-specific comparison.
• PixelDifference is not public. The report carries aggregates and frames only; the difference image shows where and how pixels differ. This also fixes the nullable TotalNormalizedDifference TODO from the test utilities.
• Difference image: mid-gray where equal; per RGB channel white or black where the actual component is above or below the expected one; fully opaque. Alpha differences are reflected in the total difference only. I kept the rendering RGB-only on purpose and am open to changing the encoding.
• Reports under threshold keep their frames, so CreateDifferenceImage() also works for matching frames and renders uniform mid-gray.

Implementation notes:

• ImageComparer.Exact is TolerantImageComparer(0, 0), matching the internal ImageComparer.Exact.
• The shared comparison and difference-image math lives in an internal ImageComparisonEngine, so both paths use one row conversion and one metric.
• AOT: Seed<TPixel>() seeds the comparison for same-type pairs through a new AotCompileImageComparison<TPixel>(), following the compile-only seeding pattern used by the encoders. Cross-type pairs follow the same discovery limits as other cross-type generic APIs.

Test coverage:

tests/ImageSharp.Tests/Comparison/ImageComparerTests.cs - 15 tests covering exact and tolerant thresholds, per-pixel threshold behavior, percentage semantics, dimension and frame-count mismatches, VerifySimilarity, multi-frame indices, difference-image rendering, the non-generic double-dispatch path, and mixed pixel formats (Rgba32 vs Rgb24).

The internal test comparers are unchanged (104 test files use them). Re-pointing them to the public API can be a follow-up.

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