Count and surface RX-queue drops on the ESP32-C5's promiscuous path

wifi_promisc_rx_cb() fed s_rx_queue with a 0-timeout xQueueSend() and never
checked whether it succeeded, so a burst of captured frames arriving faster
than rx_forward_task could drain them vanished with no counter anywhere -
none of oversizeDrops/txFailures/rxCrcErrors caught it. Added a rxQueueDrops
counter, threaded it through the STATUS heartbeat as a new trailing uint16
(old firmware/app on either side still parse fine), and surfaced it on the
CAM Pinger card.

Confirmed on the bench: flashed to the production OBU (COM3) and installed
the matching app build on the phone, then watched the counter over logcat
against obu-cam-transmistter's ~3.3 Hz beacon - it is real (0 -> 89 -> 90
across two sessions) but bursty around connect/reconnect rather than a
continuous overflow under steady single-station traffic.

obu-firmware/main/main.c

@@ -259,8 +259,13 @@ static void wifi_promisc_rx_cb(void *recv_buf, wifi_promiscuous_pkt_type_t type)
     s_cb_item.rssi = packet->rx_ctrl.rssi;
 
     // 0 timeout: never block the WiFi driver's own task waiting for queue space. xQueueSend copies
-    // the struct out before returning, so reusing s_cb_item on the next callback is fine.
-    xQueueSend(s_rx_queue, &s_cb_item, 0);
+    // the struct out before returning, so reusing s_cb_item on the next callback is fine. The
+    // return value used to go unchecked, so a full queue (rx_forward_task still draining a
+    // previous burst) silently ate frames with no counter anywhere - see
+    // serial_link_note_rx_queue_drop()'s KDoc.
+    if (xQueueSend(s_rx_queue, &s_cb_item, 0) != pdTRUE) {
+        serial_link_note_rx_queue_drop();
+    }
 }
 
 static void rx_forward_task(void *arg)