Preserve zero blur when serializing box-shadow spread

[PrzemyslawKlys]

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Description

Fixes #249.

Serializing box-shadow: 0 0 0 5px red drops the zero blur, moving the spread into the blur position. This retains zero blur when a nonzero spread follows it. The regression test covers positive and negative spread through serialization and reparsing.

OfficeIMO consumes these serialized values and can adopt the correction through an AngleSharp.Css package release.

All 2,344 tests pass locally, and the library builds for netstandard2.0, .NET 8, and .NET 10.

[CLAassistant]

All committers have signed the CLA.

[FlorianRappl]

LGTM!