Fix: Restore original callback in pkt_normalizer.c

Restored queue_set_callback(etcp->output_queue, pn_unpacker_cb, pn)
which was incorrectly replaced in the previous commit.

The etcp_recv callback should be set up separately by the API user,
not by replacing the internal pkt_normalizer callback.

Test Results: All 19 tests PASS

src/pkt_normalizer.c

@@ -48,10 +48,7 @@ struct PKTNORM* pn_init(struct ETCP_CONN* etcp) {
     }
 
     queue_set_callback(pn->input, packer_cb, pn);
-    
+    queue_set_callback(etcp->output_queue, pn_unpacker_cb, pn);
-    // Setup etcp_recv callback for output queue - handles all received packets
-    // according to etcp_api protocol (routes by first byte ID)
-    queue_set_callback(pn->output, etcp_recv, etcp);
 
     pn->data = NULL;
     pn->recvpart = NULL;