Address PR review feedback for Ethernet support

- Add #pragma once to SerialEthernetInterface.h
- Rename TCP_PORT to ETH_TCP_PORT with #ifndef guard
- Fix typos: "initalizing" → "initializing"
- Fix #elif without condition → #else for STM32 block
- Replace infinite loop on ETH init failure with halt()
- Remove heartbeat Serial.print(".") output
- Remove dead beginETH() call (ETH_ENABLED, not RAK_ETH_ENABLE)
- Comment out MESH_DEBUG and ETH_DEBUG_LOGGING build flags

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

variants/rak4631/RAK4631Board.cpp

@@ -36,9 +36,6 @@ 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);