Macro impl_product_alloc

macro_rules! impl_product_alloc {
    (
        $asset:ty,
        $rational:ty,
        $time:ty,
        $addon:ty
    ) => { ... };
}

Implements all virtual field and extension bindings for a ProductType.

Expands VirtualDynField for core discriminants (Asset, Rational, Time) and VirtualDynExtension for the addon field, using ProductProvider for bounds and schema.