Derive Macro Composite 

#[derive(Composite)]
{
    // Attributes available to this derive:
    #[composite]
    #[child]
}

Derive child-model forwarding helpers for a composite model struct.

The derive supports structs with named fields. It generates hidden aggregate helpers __composite_init, __composite_update, and __composite_subscriptions, plus one hidden <field>_view helper per field marked with #[child(...)].

Supported attributes:

• #[composite(message = ParentMsg)] on the struct to declare the parent message type
• #[child(path = "...", lift = ..., extract = ...)] on child fields to define:
  • the child path segment used for key scoping
  • a function or constructor that lifts child messages into the parent message type
  • a function that extracts a child message or returns the original parent message

Child subscriptions must remain unique after path scoping. The generated __composite_subscriptions helper panics if two child subscriptions collide.