Tighten protocol typings for strict consumers

src/codec.ts

@@ -128,7 +128,7 @@ export function decodeBuiltin(raw: string): BuiltinEnvelope {
       throw new CodecError("Missing or invalid 'type' field in envelope");
     }
 
-    return envelope as BuiltinEnvelope;
+    return envelope as unknown as BuiltinEnvelope;
   } catch (cause) {
     if (cause instanceof CodecError) {
       throw cause;