Refactor Merkle sync into bounded bidirectional rounds with atomic member updates

tests/test_chat_join_e2e.c

@@ -556,10 +556,18 @@ static int child_main(const char* role, const char* dir, int invalid_key) {
             if (!wait_join_result(inst, 8000)) {
                 fprintf(stderr, "J: no JOIN_READY\n"); goto out;
             }
+            char ready[512]; snprintf(ready, sizeof(ready), "%s/join_ready", dir);
+            wf(ready, "ready");
             rc = 0;
         }
     }
 
+    /* Локальная запись не означает доставку: участники обслуживают обмен до JOIN_READY у J. */
+    if (rc == 0 && !invalid_key && strcmp(role, "j") != 0) {
+        char ready[512]; snprintf(ready, sizeof(ready), "%s/join_ready", dir);
+        if (!wait_file(inst, ready, 8000)) { fprintf(stderr, "%s: join delivery not confirmed\n", role); rc = 1; }
+    }
+
 out:
     if (early_link || delayed_registration) rc = 1;
     fprintf(stderr, "%s: %s\n", role, rc == 0 ? "OK" : "FAIL");