Add union type support

[prozolic]

This PR adds union type support.

Support:

• union declarations
• Custom union types marked with [System.Runtime.CompilerServices.Union]
• Custom unions with typed storage (TryGetValue(out T) and optional HasValue)
• Union member provider types(IUnionMembers, IUnionMembers.Create(value))
• Class-based unions ([Union])

usage

[TomlUnion<long>]
public union IntOrText(long, string);

[TomlSerializedObject]
public partial class Setting
{
    [TomlValueOnSerialized]
    public IntOrText Num { get; set; }
}

var setting = new Setting() { Num = 123L };
using var bytes = CsTomlSerializer.Serialize(setting);

Related

#121

[Copilot]

Copilot review overview

🟡 Changes recommended

Critical generated-code failures and additional union validation, accessibility, and nullable-handling issues remain unresolved.

Review effort: Lite
Findings: 3 · 1

Open (4)

• Union formatter registration uses a mismatched nullable target type · New
• Nested generic union formatters omit containing type parameters · New
• Formatter name sanitization causes collisions between distinct case types · New
• Union member provider is accepted without verifying implementation · New

What changed in this PR

Adds source-generated serialization support for custom and C# union types.

Changes:

• Adds union analysis, metadata, diagnostics, formatter generation, and registration.
• Adds custom, generic, class-based, typed-storage, and .NET 11 union tests.
• Updates NativeAOT samples, project targets, and CI for .NET 11.

File	Summary
tests/​CsToml.Tests/​CsToml.Tests.csproj	Adds .NET 11 test targeting.
tests/​CsToml.Generator.Tests/​Generator/​UnionTest.cs	Tests custom union behavior.
tests/​CsToml.Generator.Tests/​Generator/​Net11/​UnionNet11Test.cs	Tests .NET 11 union syntax.
tests/​CsToml.Generator.Tests/​Declarations/​UnionDeclarations.cs	Defines custom union fixtures.
tests/​CsToml.Generator.Tests/​Declarations/​UnionAttributePolyfill.cs	Provides union attribute support for older targets.
tests/​CsToml.Generator.Tests/​Declarations/​Net11/​UnionNet11Declarations.cs	Defines .NET 11 union fixtures.
tests/​CsToml.Generator.Tests/​CsToml.Generator.Tests.csproj	Adds .NET 11 test targeting.
tests/​CsToml.Extensions.Configuration.Tests/​CsToml.Extensions.Configuration.Tests.csproj	Adds .NET 11 test targeting.
src/​CsToml.Generator/​UnionSymbolAnalyzer.cs	Detects unions and providers; provider validation and formatter-name collision handling require changes.
src/​CsToml.Generator/​UnionMetaFactory.cs	Builds union formatter metadata; nested unpinned cases and containing generic parameters require changes.
src/​CsToml.Generator/​UnionMeta.cs	Defines union metadata models.
src/​CsToml.Generator/​TypeMeta.cs	Integrates unions into object metadata; invalid pins can produce a duplicate diagnostic.
src/​CsToml.Generator/​TomlValueFormatterResolverEmitter.cs	Emits union formatter registrations.
src/​CsToml.Generator/​TomlSerializationKind.cs	Adds union serialization classification.
src/​CsToml.Generator/​SymbolUtility.cs	Resolves union pins and reachable types.
src/​CsToml.Generator/​Generator.cs	Emits union formatters and registration; class-union formatter typing and standalone accessibility require changes.
src/​CsToml.Generator/​FormatterTypeMetaData.cs	Classifies union serialization types; nullable unions can lose pinned-case serialization style.
src/​CsToml.Generator/​DiagnosticDescriptors.cs	Adds union diagnostics.
sandbox/​ConsoleNativeAOT/​Union.cs	Adds a NativeAOT union sample.
sandbox/​ConsoleNativeAOT/​Program.cs	Exercises union serialization.
sandbox/​ConsoleNativeAOT/​ConsoleNativeAOT.csproj	Retargets the sample to .NET 11.
.github/​workflows/​build.yml	Installs the .NET 11 SDK in CI.

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