Replace 0xFFFFFFFF sentinel with explicit getOutboundTotal()

Instead of overloading getOutboundCount() with a magic sentinel value,
add a dedicated getOutboundTotal() method to the PacketManager interface
that returns the total queue size without time filtering.

This eliminates the fragile convention that caused the regression and
makes the two operations — time-filtered count vs total count —
explicitly separate in the API.

examples/simple_room_server/MyMesh.cpp

@@ -140,7 +140,7 @@ int MyMesh::handleRequest(ClientInfo *sender, uint32_t sender_timestamp, uint8_t
   if (payload[0] == REQ_TYPE_GET_STATUS) {
     ServerStats stats;
     stats.batt_milli_volts = board.getBattMilliVolts();
-    stats.curr_tx_queue_len = _mgr->getOutboundCount(0xFFFFFFFF);
+    stats.curr_tx_queue_len = _mgr->getOutboundTotal();
     stats.noise_floor = (int16_t)_radio->getNoiseFloor();
     stats.last_rssi = (int16_t)radio_driver.getLastRSSI();
     stats.n_packets_recv = radio_driver.getPacketsRecv();