fix(network): pre-merge review fixes

- Guard the boot-time link bootstrap to observer ESP32 builds. It ran
  unconditionally in MyMesh::begin(), breaking every non-observer
  repeater/room server build (ESP32 and nRF52).
- Rename NetworkInterface -> NetworkLink (class, accessor, files).
  Arduino-ESP32 3.x ships its own NetworkInterface class and header,
  which broke the ESP32-C6 builds. Drop WiFi.setAutoConnect(), a no-op
  on 2.x and removed in 3.x.
- Refresh stored Wi-Fi credentials every bridge tick so the STA
  reconnect loop picks up `set wifi.ssid` / `set wifi.pwd` without a
  reboot, as the bridge did before the link moved out of it. Skip
  reconnects while the SSID is empty.
- Restore the "WiFi connected: <ip>" / "WiFi disconnected: reason N"
  debug lines the bridge used to print.
- Alert on Ethernet only once it has held a lease this boot or when no
  Wi-Fi is configured; Wi-Fi-only installs of an Ethernet-preferred
  image keep Wi-Fi alerts instead of reporting "Ethernet down".
- Record wifi.setup_complete only for Ethernet LAN onboarding, so
  Wi-Fi builds keep the SSID-based first-boot portal rule.
- Use seq_cst for the route-switch lock/mutation flag handshake.
- Docs: SNMP RSSI sentinel is -127; describe link-return vs medium-switch
  reconnect behavior accurately; note runtime credential pickup.
- Test: unknown keys inside a known /mqtt.json group are ignored, which
  keeps wifi.setup_complete downgrade-safe.

src/helpers/AlertReporter.cpp

@@ -6,7 +6,7 @@
 #include <stdio.h>
 #ifdef WITH_MQTT_BRIDGE
 #include "AlertFaultPolicy.h"
-#include "NetworkInterface.h"
+#include "NetworkLink.h"
 #endif
 
 // Header layout for PAYLOAD_TYPE_GRP_TXT before encryption: