Closes #3292

What

Adds a public ValueTypeAssertions<TSubject, TAssertions> base class in FluentAssertions.Primitives, giving third-party assertion authors a ready-made base for value types (structs), mirroring what ReferenceTypeAssertions<TSubject, TAssertions> already offers for reference types.

API

public abstract class ValueTypeAssertions<TSubject, TAssertions>
    where TSubject : struct
    where TAssertions : ValueTypeAssertions<TSubject, TAssertions>
{
    protected ValueTypeAssertions(TSubject? subject, AssertionChain assertionChain);

    public TSubject? Subject { get; }
    public AssertionChain CurrentAssertionChain { get; }
    protected abstract string Identifier { get; }

    public AndConstraint<TAssertions> Be(TSubject expected, string because = "", params object[] becauseArgs);
    public AndConstraint<TAssertions> NotBe(TSubject unexpected, string because = "", params object[] becauseArgs);
    public AndConstraint<TAssertions> BeNull(string because = "", params object[] becauseArgs);
    public AndConstraint<TAssertions> NotBeNull(string because = "", params object[] becauseArgs);
    public AndConstraint<TAssertions> Match(Expression<Func<TSubject, bool>> predicate, string because = "", params object[] becauseArgs);
}

The constructor and CurrentAssertionChain/WithDefaultIdentifier usage follow the same conventions already used by ReferenceTypeAssertions, BooleanAssertions, and the other primitive assertion classes in this codebase, rather than the constructor shape shown in the issue's proposal (which predates the AssertionChain pattern).

Changes

• Src/FluentAssertions/Primitives/ValueTypeAssertions.cs — new base class.
• Tests/FluentAssertions.Specs/Primitives/ValueTypeAssertionsSpecs.cs — specs covering Be, NotBe, BeNull, NotBeNull, Match, and the Equals guard, using a Coordinates struct/CoordinatesAssertions test fixture.
• Tests/Approval.Tests/ApprovedApi/**/*.verified.txt — updated via AcceptApiChanges.ps1 for the new public API.
• docs/_pages/extensibility.md — mentions the new base class alongside ReferenceTypeAssertions.
• docs/_pages/releases.md — release note entry.

Testing

• dotnet test Tests/FluentAssertions.Specs/FluentAssertions.Specs.csproj --filter "FullyQualifiedName~Primitives" — all pass (net47/net6.0/net8.0).
• dotnet test Tests/Approval.Tests/Approval.Tests.csproj — all pass after accepting the API change.