Update SVG upstream and complete SkiaSharp 3 AOT cleanup

[wieslawsoltes]

Summary

This PR updates the SVG submodule to the latest upstream commit carrying the trim/AOT annotation cleanup and completes the downstream Svg.Skia changes needed to build against SkiaSharp 3 without runtime sampling reflection.

What Changed

• Updated externals/SVG to upstream commit 853a9ae8, including the SVG library trim/AOT annotation cleanup from Remove DynamicallyAccessedMemberTypes to improve trim/aot compatibility svg-net/SVG#1211.
• Moved central SkiaSharp package versions to stable 3.119.2.
• Added build/SkiaSharp.Avalonia.props so direct SkiaSharp references in Avalonia sample/test apps stay aligned with the Avalonia.Skia 12.0.0 SkiaSharp 3.119.3-preview.1.1 dependency.
• Removed the old SkiaSharp.v3.props and SkiaSharp.Native.v3.props split and normalized project imports.
• Removed the reflected SkiaSharpSamplingOptionsApi bridge from SkiaModel.
• Replaced image drawing, image-filter, and downsampling paths with statically referenced SkiaSharp 3 SKSamplingOptions APIs.
• Updated generated Skia C# output for image filters to emit SkiaSharp 3 sampling options.
• Removed remaining RequiresUnreferencedCode and DynamicallyAccessedMembers annotations from the reviewed runtime path.
• Made animation value conversion avoid TypeDescriptor.GetConverter(Type) by reusing per-property converters captured from SVG descriptors and by handling common primitive/unit conversions directly.
• Added raw JavaScript DOM attribute state preservation so DOM-assigned values such as SVGAngle.valueAsString keep their original unit spelling and survive DeepCopy().
• Reworked generated SvgML enum bridge parsing/formatting to avoid TypeDescriptor.GetConverter while preserving SVG casing such as visiblePainted, whenNotActive, color-dodge, and hueRotate.
• Added regression coverage for JavaScript DOM raw angle/empty-attribute state and SvgML enum bridge formatting/parsing.

Why

SkiaSharp 3 exposes the sampling API directly. Keeping the old runtime reflection bridge required AOT annotations and retained a trim-sensitive path. Moving to SkiaSharp 3 as the baseline lets Svg.Skia use the real static API surface while preserving framework-specific package constraints for Avalonia.

The SVG upstream update also changed enum/string conversion behavior in ways that affected JavaScript DOM compatibility. The new raw DOM attribute side table keeps script-observable string values distinct from typed model conversions.

Validation

• dotnet format Svg.Skia.slnx --no-restore
• dotnet build Svg.Skia.slnx -c Release
• dotnet test Svg.Skia.slnx -c Release
• dotnet build src/Svg.Controls.Skia.Maui/Svg.Controls.Skia.Maui.csproj -c Release
• dotnet build src/SvgML.Maui/SvgML.Maui.csproj -c Release
• git diff --check
• rg -n "DynamicallyAccessedMembers|RequiresUnreferencedCode|RequiresDynamicCode|SkiaSharpSamplingOptionsApi" src externals/SVG -g '*.cs'

Notes

The full solution build/test still reports existing warnings only: known package advisories, nullable warnings, and SkiaSharp 3 text API obsoletions. The reviewed runtime paths no longer contain RequiresUnreferencedCode, DynamicallyAccessedMembers, or the SkiaSharp sampling reflection shim.

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