feat(thinknode-m7): make the MQTT observer envs Ethernet-preferred

Fold the CH390 overlay and NETWORK_PREFER_ETHERNET into
ThinkNode_M7_{repeater,room_server}_observer_mqtt and drop the separate
*_observer_mqtt_ethernet twins. The env names (and so the OTA manifests)
are unchanged, so existing M7 observers OTA into the Ethernet-preferred
image; without a cable they select stored Wi-Fi after a brief boot probe.

MQTT_IMPLEMENTATION.md

@@ -225,11 +225,14 @@ pio run -e ThinkNode_M7_repeater_observer_mqtt
 pio run -e ThinkNode_M7_room_server_observer_mqtt
 ```
 
-**ThinkNode M7 — WiFi only:** the M7 has an onboard CH390 Ethernet controller, and
-`ThinkNode_M7_companion_radio_ethernet` uses it, but the MQTT bridge's link
-management is bound to the WiFi station API, so the observer envs uplink over WiFi.
-See `UPSTREAM_BUGS.md` for the Ethernet gap. The board has PSRAM, so these builds
-get neighbors publication (`WITH_MQTT_NEIGHBORS`) automatically.
+**ThinkNode M7 — Ethernet preferred, WiFi fallback:** the observer envs use the onboard
+CH390 Ethernet when it has a DHCP lease at boot, otherwise the stored WiFi network, and
+switch between the two at runtime (`get link.status`, `get link.diag`). Nodes without a
+cable behave like WiFi observers after a brief (about 1 s) Ethernet probe at boot. The DHCP hostname is
+`meshcore-<node name>` on either medium. A first boot on Ethernet with no WiFi configured
+opens WebConfig on the LAN with a one-time login code printed on serial (and shown on
+the display), and requires replacing the admin password. The board has PSRAM, so these
+builds get neighbors publication (`WITH_MQTT_NEIGHBORS`) automatically.
 
 **TLora naming:** The env prefix `LilyGo_TLora_V2_1_1_6` is LilyGo’s **T-LoRa V2.1–1.6** board (SX1276); PlatformIO selects **`ttgo-lora32-v1`** (TTGO LoRa32 V1.0). **MQTT observer** envs extend a slim base **without** `sensor_base` so the image fits `min_spiffs`; **all other** `LilyGo_TLora_V2_1_1_6_*` targets still use optional I2C environmental sensors as before. The **`lilygo_tlora_c6`** variant is separate hardware (ESP32-C6).
 

variants/thinknode_m7/platformio.ini

@@ -191,10 +191,9 @@ extends = ThinkNode_M7
 build_src_filter = ${ThinkNode_M7.build_src_filter}
   +<../examples/kiss_modem/>
 
-; Wi-Fi remains the default observer transport. Ethernet twins below select the
-; onboard CH390 through the Ethernet-preferred network manager without enabling the legacy CLI
-; transport. The board has PSRAM, so MAX_NEIGHBOURS enables
-; WITH_MQTT_NEIGHBORS (see MQTTBridge.h).
+; MQTT observers prefer the onboard CH390 Ethernet and fall back to Wi-Fi, without
+; enabling the legacy Ethernet CLI transport (no ETHERNET_ENABLED). The board has
+; PSRAM, so MAX_NEIGHBOURS enables WITH_MQTT_NEIGHBORS (see MQTTBridge.h).
 [env:ThinkNode_M7_repeater_observer_mqtt]
 extends = ThinkNode_M7
 extra_scripts =
@@ -204,6 +203,8 @@ board_ssl_cert_source = adafruit-full
 board_build.embed_files = src/certs/x509_crt_bundle.bin
 build_flags =
   ${ThinkNode_M7.build_flags}
+  ${ThinkNode_M7_ch390.build_flags}
+  -D NETWORK_PREFER_ETHERNET=1
   -D ADVERT_NAME='"MQTT Observer"'
   -D ADVERT_LAT=0.0
   -D ADVERT_LON=0.0
@@ -231,6 +232,7 @@ build_src_filter = ${ThinkNode_M7.build_src_filter}
   +<../examples/simple_repeater>
 lib_deps =
   ${ThinkNode_M7.lib_deps}
+  ${ThinkNode_M7_ch390.lib_deps}
   ${esp32_ota.lib_deps}
   elims/PsychicMqttClient@^0.2.4
   bblanchon/ArduinoJson @ 7.4.3
@@ -239,17 +241,6 @@ lib_deps =
   paulstoffregen/Time@1.6.1
   0neblock/SNMP_Agent
 
-[env:ThinkNode_M7_repeater_observer_mqtt_ethernet]
-extends = env:ThinkNode_M7_repeater_observer_mqtt
-build_flags =
-  ${env:ThinkNode_M7_repeater_observer_mqtt.build_flags}
-  ${ThinkNode_M7_ch390.build_flags}
-  -D NETWORK_PREFER_ETHERNET=1
-build_src_filter = ${env:ThinkNode_M7_repeater_observer_mqtt.build_src_filter}
-lib_deps =
-  ${env:ThinkNode_M7_repeater_observer_mqtt.lib_deps}
-  ${ThinkNode_M7_ch390.lib_deps}
-
 [env:ThinkNode_M7_room_server_observer_mqtt]
 extends = ThinkNode_M7
 extra_scripts =
@@ -259,6 +250,8 @@ board_ssl_cert_source = adafruit-full
 board_build.embed_files = src/certs/x509_crt_bundle.bin
 build_flags =
   ${ThinkNode_M7.build_flags}
+  ${ThinkNode_M7_ch390.build_flags}
+  -D NETWORK_PREFER_ETHERNET=1
   -D ADVERT_NAME='"ThinkNode M7 Room Observer"'
   -D ADVERT_LAT=0.0
   -D ADVERT_LON=0.0
@@ -287,6 +280,7 @@ build_src_filter = ${ThinkNode_M7.build_src_filter}
   +<../examples/simple_room_server>
 lib_deps =
   ${ThinkNode_M7.lib_deps}
+  ${ThinkNode_M7_ch390.lib_deps}
   ${esp32_ota.lib_deps}
   elims/PsychicMqttClient@^0.2.4
   bblanchon/ArduinoJson @ 7.4.3
@@ -294,14 +288,3 @@ lib_deps =
   JChristensen/Timezone
   paulstoffregen/Time@1.6.1
   0neblock/SNMP_Agent
-
-[env:ThinkNode_M7_room_server_observer_mqtt_ethernet]
-extends = env:ThinkNode_M7_room_server_observer_mqtt
-build_flags =
-  ${env:ThinkNode_M7_room_server_observer_mqtt.build_flags}
-  ${ThinkNode_M7_ch390.build_flags}
-  -D NETWORK_PREFER_ETHERNET=1
-build_src_filter = ${env:ThinkNode_M7_room_server_observer_mqtt.build_src_filter}
-lib_deps =
-  ${env:ThinkNode_M7_room_server_observer_mqtt.lib_deps}
-  ${ThinkNode_M7_ch390.lib_deps}