Migrate TagExpressions to source-generated regexes (multi-target netstandard2.0 + net8.0)

[Copilot]

Replaces the static Regex fields and static constructor in TagExpressions.cs with source-generated regexes on net8.0+, while retaining compiled Regex fallbacks for netstandard2.0.

TagExpressions.cs

• Made partial
• #if NET8_0_OR_GREATER: [GeneratedRegex]-decorated static partial methods (compile-time generated, zero-allocation)
• #else: static readonly Regex fields with RegexOptions.Compiled exposed through matching static Regex Foo() methods — identical call-site signature across both targets

#if NET8_0_OR_GREATER
    [GeneratedRegex(@"<[^\s>]*\b(name|value-of)\b[^>]*/>")]
    public static partial Regex NameValueOf();
#else
    static readonly Regex _nameValueOf = new Regex(@"<[^\s>]*\b(name|value-of)\b[^>]*/>", RegexOptions.Compiled);
    public static Regex NameValueOf() => _nameValueOf;
#endif

The AllSchematron pattern hardcodes the Regex.Escape-d values of Schema.LegacyNamespace / Schema.IsoNamespace (required since [GeneratedRegex] demands a compile-time constant); a comment flags the dependency.

Schematron.csproj

• TargetFramework → TargetFrameworks: netstandard2.0;net8.0
• PolySharp restored, scoped to netstandard2.0 only (not needed on net8.0)

Call sites

All four call sites (LogFormatter, XmlFormatter, Config, Test) already use TagExpressions.Foo() method-call syntax from the initial migration step — no further changes needed.

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[kzu]

🧪 Details on Ubuntu 24.04.4 LTS

from retest v1.1.0 on .NET 10.0.5 with 💜 by @devlooped