Fix Option<String> pattern match double-dereference in C codegen

LuxString is typedef char* but the codegen treated it as a struct type,
generating *(LuxString*)(field0) instead of (LuxString)(field0). This
caused a heap-buffer-overflow on any Option<String> pattern match since
it read the string contents as a memory address.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/codegen/c_backend.rs

@@ -5316,11 +5316,12 @@ impl CBackend {
                     if Self::is_primitive_c_type(&actual_type) {
                         // For primitive types stored as boxed void*, dereference
                         self.writeln(&format!("{} {} = *({}*)({});", actual_type, var_name, actual_type, c_expr));
-                    } else if !actual_type.ends_with('*') && actual_type != "void" {
+                    } else if actual_type == "LuxString" || actual_type.ends_with('*') || actual_type == "void" {
+                        // Pointer types (including LuxString which is typedef char*): simple cast
+                        self.writeln(&format!("{} {} = ({})({});", actual_type, var_name, actual_type, c_expr));
+                    } else {
                         // Struct types: cast to pointer and dereference
                         self.writeln(&format!("{} {} = *({}*)({});", actual_type, var_name, actual_type, c_expr));
-                    } else {
-                        self.writeln(&format!("{} {} = ({})({});", actual_type, var_name, actual_type, c_expr));
                     }
                     self.var_types.insert(var_name.clone(), actual_type);
                 } else if actual_type.ends_with('*') && actual_type != "void*" {