invalid return naming policy

@naming belongs on a struct, enum, or newtype declaration, once per declaration. It accepts only the named returns argument, containing a non-empty literal array of .SnakeCase and/or .PascalCase, with no repeated styles.

@naming(returns: [.SnakeCase, .PascalCase])
pub struct Node { text: str }

fn Greeting() { Node.new(text: "Hello") }
fn build_node() -> Node { Node.new(text: "Hello") }

Choose just .SnakeCase or just .PascalCase to permit only that style. The labels are built-in metadata; no enum declaration or import is required. The policy follows inferred returns, transparent aliases, and imports.

Move a misplaced policy to the return type, remove extra arguments or duplicate policies, and correct the style list. Unknown styles, strings, payloads, and computed arrays are rejected. Invalid declarations install no partial policy.