PTP: drop the "Preferred TimeTransmitter" role

It behaved exactly like auto (clock class 248); the only difference was a
priority1 suggestion in the page. Roles are now slave|auto; to prefer this
device, use auto with a low priority1. A stored "master" is converted to
auto at boot (new cfg_set_string, like cfg_set_number).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

components/aes67_ptp/ptp_clock.c

@@ -622,8 +622,8 @@ static void load_config(void)
     s.cfg_log_sync = cJSON_GetObjectItem(c, "log_sync")->valueint;
     s.cfg_log_announce = cJSON_GetObjectItem(c, "log_announce")->valueint;
     s.cfg_log_dreq = cJSON_GetObjectItem(c, "log_delay_req")->valueint;
-    // Roles (docs): slave = clockClass 255, never transmits; auto/master = 248 with the configured
-    // priorities (auto 250/250 by default, master e.g. p1 100).
+    // Roles (docs): slave = clockClass 255, never transmits; auto = 248 with the configured priorities
+    // (250/250 by default: loses to any real GM; a lower priority1 prefers this device).
     s.slave_only = strcmp(cJSON_GetObjectItem(c, "role")->valuestring, "slave") == 0;
     s.hybrid = strcmp(cJSON_GetObjectItem(c, "mode")->valuestring, "hybrid") == 0;
     s.own_class = s.slave_only ? 255 : 248;
@@ -711,7 +711,7 @@ static void ptp_task(void *arg)
             int tos2 = s.dscp << 2;
             setsockopt(s.gen, IPPROTO_IP, IP_TOS, &tos2, sizeof(tos2));
             ESP_LOGI(TAG, "config applied: role %s, mode %s, p1 %u p2 %u, domain %u",
-                     s.slave_only ? "slave" : "auto/master", s.hybrid ? "hybrid" : "multicast",
+                     s.slave_only ? "slave" : "auto", s.hybrid ? "hybrid" : "multicast",
                      s.own.p1, s.own.p2, s.domain);
             // Re-run the decision: a foreign GM worse than our new dataset is dropped; as
             // TimeTransmitter with role slave we stop.