Add native Ethernet support for RAK4631 repeater, room server, and companion

Add W5100S Ethernet adapter support for RAK4631-based firmware, enabling
TCP CLI access on port 23 as an alternative to BLE/Serial connections.

- New SerialEthernetInterface for nRF52 with DHCP, reconnection handling,
  and shared WB_IO2 power pin management with GPS module
- Ethernet build targets for repeater, room server, and companion firmware
- Prevent GPS from toggling WB_IO2 when Ethernet module is active
- CI build check for all three ETH firmware targets

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

variants/rak4631/RAK4631Board.cpp

@@ -36,6 +36,10 @@ void RAK4631Board::begin() {
   pinMode(PIN_USER_BTN_ANA, INPUT_PULLUP);
 #endif
 
+#ifdef RAK_ETH_ENABLE
+  beginETH();
+#endif
+
 #if defined(PIN_BOARD_SDA) && defined(PIN_BOARD_SCL)
   Wire.setPins(PIN_BOARD_SDA, PIN_BOARD_SCL);
 #endif

variants/rak4631/platformio.ini

@@ -46,6 +46,26 @@ build_src_filter = ${rak4631.build_src_filter}
   +<helpers/ui/SSD1306Display.cpp>
   +<../examples/simple_repeater>
 
+[env:RAK_4631_repeater_eth]
+extends = rak4631
+build_flags =
+  ${rak4631.build_flags}
+  -D DISPLAY_CLASS=SSD1306Display
+  -D ADVERT_NAME='"RAK4631 Repeater ETH"'
+  -D ADVERT_LAT=0.0
+  -D ADVERT_LON=0.0
+  -D ADMIN_PASSWORD='"password"'
+  -D MAX_NEIGHBOURS=50
+  -D ETH_ENABLED=1
+  -D MESH_DEBUG=1
+build_src_filter = ${rak4631.build_src_filter}
+  +<helpers/ui/SSD1306Display.cpp>
+  +<../examples/simple_repeater>
+lib_deps =
+  ${rak4631.lib_deps}
+  # renovate: datasource=github-tags depName=RAK13800-W5100S packageName=RAKWireless/RAK13800-W5100S
+  https://github.com/RAKWireless/RAK13800-W5100S/archive/1.0.2.zip
+
 [env:RAK_4631_repeater_bridge_rs232_serial1]
 extends = rak4631
 build_flags =
@@ -108,6 +128,26 @@ build_src_filter = ${rak4631.build_src_filter}
   +<helpers/ui/SSD1306Display.cpp>
   +<../examples/simple_room_server>
 
+[env:RAK_4631_room_server_eth]
+extends = rak4631
+build_flags =
+  ${rak4631.build_flags}
+  -D DISPLAY_CLASS=SSD1306Display
+  -D ADVERT_NAME='"Test Room ETH"'
+  -D ADVERT_LAT=0.0
+  -D ADVERT_LON=0.0
+  -D ADMIN_PASSWORD='"password"'
+  -D ROOM_PASSWORD='"hello"'
+  -D ETH_ENABLED=1
+  -D MESH_DEBUG=1
+build_src_filter = ${rak4631.build_src_filter}
+  +<helpers/ui/SSD1306Display.cpp>
+  +<../examples/simple_room_server>
+lib_deps =
+  ${rak4631.lib_deps}
+  # renovate: datasource=github-tags depName=RAK13800-W5100S packageName=RAKWireless/RAK13800-W5100S
+  https://github.com/RAKWireless/RAK13800-W5100S/archive/1.0.2.zip
+
 [env:RAK_4631_companion_radio_usb]
 extends = rak4631
 board_build.ldscript = boards/nrf52840_s140_v6_extrafs.ld
@@ -129,6 +169,37 @@ lib_deps =
   ${rak4631.lib_deps}
   densaugeo/base64 @ ~1.4.0
 
+
+[env:RAK_4631_companion_radio_eth]
+extends = rak4631
+board_build.ldscript = boards/nrf52840_s140_v6.ld
+board_upload.maximum_size = 712704
+build_unflags =
+  -D EXTRAFS=1
+build_flags =
+  ${rak4631.build_flags}
+  -I examples/companion_radio/ui-new
+  -D PIN_USER_BTN=9
+  -D PIN_USER_BTN_ANA=31
+  -D DISPLAY_CLASS=SSD1306Display
+  -D MAX_CONTACTS=350
+  -D MAX_GROUP_CHANNELS=40
+  -D ETH_ENABLED=1
+; NOTE: DO NOT ENABLE -->  -D MESH_PACKET_LOGGING=1
+; NOTE: DO NOT ENABLE -->  -D MESH_DEBUG=1
+  -D MESH_DEBUG=1
+  -D ETH_DEBUG_LOGGING=1
+build_src_filter = ${rak4631.build_src_filter}
+  +<../examples/companion_radio/*.cpp>
+  +<../examples/companion_radio/ui-new/*.cpp>
+  +<helpers/nrf52/SerialEthernetInterface.cpp>
+lib_deps =
+  ${rak4631.lib_deps}
+  densaugeo/base64 @ ~1.4.0
+  # renovate: datasource=github-tags depName=RAK13800-W5100S packageName=RAKWireless/RAK13800-W5100S
+  https://github.com/RAKWireless/RAK13800-W5100S/archive/1.0.2.zip
+
+
 [env:RAK_4631_companion_radio_ble]
 extends = rak4631
 board_build.ldscript = boards/nrf52840_s140_v6_extrafs.ld
@@ -192,4 +263,4 @@ build_flags =
 build_src_filter = ${rak4631.build_src_filter}
   +<../examples/kiss_modem/>
 lib_deps =
-  ${rak4631.lib_deps}
+  ${rak4631.lib_deps}