Function generate_c_header 

pub fn generate_c_header(
    stmts: &[Stmt<'_>],
    module_name: &str,
    interner: &Interner,
    registry: &TypeRegistry,
) -> String

Generate the C header (.h) content for all C-exported functions.

Includes:

• Runtime types (logos_status_t, logos_handle_t)
• Runtime functions (logos_get_last_error, logos_clear_error, logos_free_string)
• Value-type struct definitions
• Exported function declarations
• Accessor function declarations for reference types