Improve W5500 Ethernet configuration and diagnostics

[BanjoR]

Automatic issue closing

Fixes #308
Fixes #313

Issue #375 is broader network/DHCP behavior; this PR does not claim to close that full report.

Summary

Refresh W5100/W5500 Ethernet configuration and diagnostics for wired CommandStation builds:

• add platform-gated chip-select selection through ETHERNET_CS and ETHERNET_CS_PIN
• add the ESP32-Ethernet PlatformIO environment with configurable SPI and reset pins
• add WIZnet hardware, link, DHCP, hostname, and IP diagnostics
• document wired-network configuration, dependency limitations, and a reproducible W5500 bench procedure

Bounded scope and exclusions

The exact changed-file scope is limited to:

• EthernetInterface.cpp
• EthernetInterface.h
• config.example.h
• defines.h
• platformio.ini

This change does not vendor or modify the Arduino Ethernet dependency, add Wi-Fi behavior, change the protocol/API, or include the ESP32 OTA/custom-server work from PR #525. The wired environments set ENABLE_WIFI=false and ENABLE_ETHERNET=true; the existing configuration also rejects simultaneous Wi-Fi and Ethernet. WIFI_SSID and WIFI_PASSWORD are not used for Ethernet.

The official Arduino Ethernet dependency requests DHCP when IP_ADDRESS is omitted but hard-codes its DHCP hostname option. ETHERNET_HOSTNAME is applied to DCC-EX mDNS/native STM32 naming and reported at startup; a custom W5500 DHCP hostname requires a dependency API/library update: https://github.com/arduino-libraries/Ethernet/blob/master/src/Dhcp.cpp

References

Authoritative issues:

• Issue #308
• Issue #313
• Issue #375

Superseded or incomplete PRs:

• PR #316
• PR #317
• PR #525

Validation evidence

Host audit

• Clean checkout on branch codex/commandstation-ex-commandstation-esp32-ethernet; origin is BanjoR/CommandStation-EX, upstream is DCC-EX/CommandStation-EX, and upstream/master is 0ad3080.
• PR head is 687a8c2, with upstream/master as its ancestor.
• Worktree, index, untracked files, and ignored generated outputs were clean after validation; the public PR file list is exactly the five files above.
• git diff --check upstream/master...HEAD: PASS.
• Focused static assertions for diagnostics, configurable SPI/CS/reset, Wi-Fi exclusion, PlatformIO environments/flags, DHCP/hostname limitation, and bench documentation: PASS.
• Repository CI-equivalent generated-config build, python -m platformio run for mega2560: PASS. RAM 19.0% (1560/8192); Flash 38.3% (97226/253952).
• Focused W5500 Mega build, python -m platformio run -e mega2560-eth: PASS. RAM 27.5% (2251/8192); Flash 42.1% (106848/253952).
• Embedded Ztest.cpp compiled in both Mega builds. The repository has no standalone host/unit-test target, and no runtime test was executed.
• PASS - Exact default python -m platformio run in a clean task checkout with an isolated PlatformIO core completed for all five configured environments: mega2560, ESP32, Nucleo-F411RE, Nucleo-F446RE, and Nucleo-F429ZI.
• PASS - Focused python -m platformio run -e ESP32-Ethernet after the ESP32 Server API compatibility adapter: 22.15 seconds; RAM 27,304/327,680 (8.3%); Flash 497,301/1,310,720 (37.9%).
• No standalone CMake/C++ compiler or formatter was available outside PlatformIO; git diff --check was the available whitespace/format gate.

GitHub Actions and required checks

• PR #541 is OPEN and ready for review, based on master at 0ad3080 and head 687a8c2.
• The upstream PR status-check rollup remains empty because the repository firmware workflow is push-triggered; the successful fork run below is the available exact-head firmware CI evidence.
• The upstream CI workflow is configured as on: [push], installs PlatformIO, copies config.example.h to config.h, and runs python -m platformio run; it does not declare a pull_request trigger, so no firmware CI check is attached to this cross-repository PR.
• Exact-head fork CI: PASS - CI run 31995620650 completed successfully for head 687a8c2, including the Compile Command Station (AVR) job.
• Upstream PR status remains empty because the repository firmware workflow is push-triggered; the fork run above is the available exact-head firmware CI evidence. Ancillary Docs and Label sponsors runs are not used as firmware evidence.

Hardware validation

Hardware validation: Not run—no hardware available

Maintainer bench procedure:

1. Use a Mega2560 or ESP32 with a W5500 module. Connect power and ground, SPI SCK/MISO/MOSI, the configured CS pin, and the optional ESP32 reset pin according to the module’s electrical requirements; connect the RJ45 to a DHCP-enabled LAN.
2. Build mega2560-eth or ESP32-Ethernet with a generated config.h. Exercise both the documented default and a non-default CS setting (ETHERNET_CS on Mega or ETHERNET_CS_PIN on ESP32), adjusting the SPI/reset pins when required by the wiring.
3. Capture startup diagnostics and require W5500 hardware detection, the expected CS/SPI/reset values, link ON, DHCP request/success, hostname reporting, and a non-zero IP address.
4. Connect a client to IP_PORT (default 2560) and verify a DCC-EX command/reply round trip.
5. Disconnect and reconnect the Ethernet cable. Require link/DHCP failure diagnostics while disconnected and successful link/IP recovery after reconnection.