metadata times: -U/--atimes, -N/--crtimes, --open-noatime, -O/-J

Capture+transmit source atime (pre-read stat; O_NOATIME sender guard) and birth
time (statx STATX_BTIME); receiver restores atime with mtime (crtime not settable
portably -> transmitted, explicitly not applied). --open-noatime is client-only.
-O/-J documented as accepted no-ops (FastSync never preserves dir/symlink times).
Wire: metadata frame gains atime/crtime val+sec+nsec; PROTOCOL_VERSION
2.11.0->2.12.0. Review fixes: gate atime capture to Linux (no epoch clobber on
non-Linux), close fd on fdopen failure, honest -O/-J status (Compat no-op).

tests/test_config.c

@@ -911,6 +911,51 @@ static void test_config_receive_rejects_invalid_checksum_algo() {
 /* The identity-mapping fields (--numeric-ids / --usermap / --groupmap /
    --chown) cross the config wire unchanged: the receiver needs them to apply
    ownership with the same policy the client requested. */
+static void test_config_metadata_times_wire_roundtrip() {
+  if (is_running_under_valgrind())
+    return;
+  Config* send_cfg = config_create();
+  EXPECT_NOT_NULL(send_cfg);
+  send_cfg->send_directory = str_dup("/send/src");
+  send_cfg->receive_root_directory = str_dup("/send/dst");
+  send_cfg->preserve_atimes = true;
+  send_cfg->preserve_crtimes = true;
+  send_cfg->omit_dir_times = true;
+  send_cfg->omit_link_times = true;
+  /* --open-noatime is client-only and must NOT cross the wire. */
+  send_cfg->open_noatime = true;
+
+  int p[2];
+  EXPECT_EQ_INT(socketpair(AF_UNIX, SOCK_STREAM, 0, p), 0);
+  io_set_fds(p[0], p[1]);
+  io_set_bwlimit(0);
+  pid_t pid = fork();
+  if (pid == 0) {
+    close(p[1]);
+    io_set_fds(p[0], p[0]);
+    Config* recv = config_receive(p[0]);
+    bool ok = recv != NULL;
+    if (ok) {
+      ok = recv->preserve_atimes && recv->preserve_crtimes && recv->omit_dir_times &&
+           recv->omit_link_times && !recv->open_noatime;
+    }
+    config_delete(recv);
+    close(p[0]);
+    close(p[1]);
+    _exit(ok ? 0 : 1);
+  } else {
+    close(p[0]);
+    io_set_fds(p[1], p[1]);
+    bool sent = config_send(p[1], send_cfg);
+    int status;
+    waitpid(pid, &status, 0);
+    close(p[1]);
+    config_delete(send_cfg);
+    EXPECT_TRUE(sent);
+    EXPECT_TRUE(WIFEXITED(status) && WEXITSTATUS(status) == 0);
+  }
+}
+
 static void test_config_identity_wire_roundtrip() {
   if (is_running_under_valgrind())
     return;
@@ -1068,6 +1113,7 @@ void test_config() {
     test_config_receive_rejects_invalid_checksum_algo();
     test_config_identity_wire_roundtrip();
     test_config_receive_rejects_invalid_identity();
+    test_config_metadata_times_wire_roundtrip();
     test_config_preallocate_wire_roundtrip();
   }
   test_config_delete_timing_early_helper();