Add PolyType support for efficient serialization integration

[Copilot]

This PR implements support for PolyType integration as requested in issue SteveDunn#834. The implementation conditionally emits additional code when PolyType.TypeShapeAttribute is available in the compilation, enabling seamless integration between Vogen value objects and PolyType-based serialization libraries like Nerdbank.MessagePack.

Problem Statement

PolyType is a high-performance type modeling library that uses source generation to expose type graphs at runtime. However, PolyType's source generator cannot see code emitted by Vogen's source generator, creating a compatibility issue. This forces users to either:

• Use separate assemblies for data models and serialization (suboptimal)
• Accept inefficient serialized schemas with unnecessary wrapper layers

Solution

When PolyType.TypeShapeAttribute is detected in the compilation, Vogen now automatically generates:

1. TypeShape attribute on the partial value object declaration
2. PolyTypeMarshaler nested class that implements IMarshaler<T, TUnderlying>

This enables PolyType to efficiently serialize/deserialize the underlying primitive value directly, bypassing the wrapper type and producing optimal schemas.

Example

For a value object like:

[ValueObject<int>]
public partial struct CustomerId { }

When PolyType is available, Vogen now generates:

[global::PolyType.TypeShapeAttribute(Marshaler = typeof(PolyTypeMarshaler), Kind = global::PolyType.TypeShapeKind.None)]
public partial struct CustomerId
{
    [global::System.ComponentModel.EditorBrowsable(global::System.ComponentModel.EditorBrowsableState.Never)]
    public class PolyTypeMarshaler : global::PolyType.IMarshaler<CustomerId, int>
    {
        int global::PolyType.IMarshaler<CustomerId, int>.Marshal(CustomerId value) => value.Value;
        CustomerId global::PolyType.IMarshaler<CustomerId, int>.Unmarshal(int value) => From(value);
    }
}

Implementation Details

• Conditional Generation: Code only appears when PolyType.TypeShapeAttribute is found in the compilation
• Zero Configuration: No user flags or attributes required - automatically detects PolyType availability
• Universal Support: Works with all Vogen type declarations (class, struct, record class, record struct)
• Type Safety: Uses proper global:: prefixes and follows existing Vogen patterns
• Format Agnostic: Works with any PolyType-based serialization library, not just MessagePack

Benefits

• Enables high-performance serialization with PolyType without requiring separate assemblies
• Produces optimal serialized schemas by marshaling the underlying primitive directly
• Supports all current and future PolyType use cases
• Maintains full backward compatibility - no breaking changes

This resolves the core incompatibility between PolyType and Vogen source generators, enabling developers to use both libraries together seamlessly for efficient, type-safe serialization.

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