Add RAK13800 Ethernet Companion targets with multi-client TCP support

[mathisono]

This PR is a follow-up to the Ethernet support merged in #1983.

It adds the RAK13800 Ethernet Companion board-support layer and upgrades the Companion Ethernet transport from single-client behavior to multi-client framed Companion TCP.

Headline changes

Multi-client framed Companion TCP

This PR updates SerialEthernetInterface so multiple Companion TCP clients can remain connected at the same time.

The transport now supports:

• MAX_ETH_CLIENTS, defaulting to 3
• per-client connection slots
• per-client receive parser state
• round-robin receive polling
• _last_rx tracking for command response routing
• broadcast delivery for push frames
• slot-only disconnect cleanup
• rejection of only the new client when all slots are full

This avoids the old failure mode where a second connection or incoming packet could close the live Companion socket.

Framed Companion API preserved

This is still MeshCore Companion framing, not CLI/raw-line mode:

client -> radio: '<' + uint16_le(length) + payload
radio -> client: '>' + uint16_le(length) + payload

This PR intentionally does not enable raw line mode for the Companion target.

RAK13800 board-support targets

Adds canonical RAK13800 Ethernet Companion targets:

• RAK_RAK13800_companion_radio_eth
• RAK_RAK13800_companion_radio_eth_static_diag

Keeps compatibility aliases for older names:

• RAK_4631_companion_radio_eth_clean
• RAK_4631_companion_radio_eth_static_diag
• RAK_4631_companion_radio_ethernet

Crow / gateway settings

The RAK13800 Companion target uses:

• ETHERNET_ENABLED=1
• ETHERNET_TCP_PORT=4403
• FORCE_CLIENT_REPEAT=0
• MAX_CONTACTS=128
• framed Companion packets
• no raw-line CLI mode

The static diagnostic target adds:

• IP: 10.245.94.47
• gateway: 10.245.94.33
• DNS: 10.245.94.33
• subnet: 255.255.255.224
• debug logging enabled

Device model

Ethernet builds report:

RAK 4631 / RAK13800 + W5100S

Normal RAK4631 builds continue to report the normal RAK4631 model string.

What this is not

• Not the repeater CLI target
• Not raw-line Ethernet CLI
• Not MQTT
• Not Meshtastic

This is the framed MeshCore Companion API over Ethernet TCP, with multi-client support for RAK4631 + RAK13800/W5100S hardware.

Validation

Added Companion TCP smoke-test scripts for:

• normal framed Companion TCP
• multi-client Companion TCP behavior

Expected validation:

pio run -e RAK_RAK13800_companion_radio_eth
pio run -e RAK_RAK13800_companion_radio_eth_static_diag

python3 scripts/meshcore_companion_tcp_smoke_test.py 10.245.94.47 4403
python3 scripts/meshcore_companion_multi_client_test.py 10.245.94.47 4403

Expected static diagnostic hardware result:

• device reachable at 10.245.94.47/27
• TCP 4403 open
• APP_START receives a response
• DEVICE_QUERY receives response code 0x0d
• two Companion TCP clients can remain connected simultaneously

[mathisono]

Progress summary:

This PR replaces the earlier target-name-only framing from #2929 with a clearer branch and PR scope: RAK13800 Ethernet Companion board support with multi-client framed Companion TCP.

What has been completed:

• Rebased onto dev after Support for RAK ethernet module #1983 merged.

• Built on top of the merged RAK Ethernet foundation from Support for RAK ethernet module #1983.

• Added canonical RAK13800 Ethernet Companion targets:

• RAK_RAK13800_companion_radio_eth

• RAK_RAK13800_companion_radio_eth_static_diag

• Kept compatibility aliases for the older RAK_4631_* companion Ethernet target names.

• Preserved framed MeshCore Companion TCP packets.

• Did not enable raw-line CLI mode for the Companion target.

• Updated the Companion Ethernet transport to support multiple simultaneous clients.

• Kept new TCP client acceptance on server.accept(), not server.available().

• Set the RAK13800 Companion TCP target to ETHERNET_TCP_PORT=4403 for Crow/gateway compatibility.

• Added static diagnostic networking at 10.245.94.47/27.

• Set FORCE_CLIENT_REPEAT=0 for gateway stability.

• Updated Ethernet build manufacturer/model reporting to RAK 4631 / RAK13800 + W5100S.

• Added Companion TCP smoke-test scripts, including a multi-client test.

• Updated README, flashing, validation, and FAQ docs.

Transport verification completed:

• server.available in src/helpers/nrf52, examples, and variants: no hits.
• server.accept is present in SerialEthernetInterface.cpp and separately in EthernetCLI.h.
• ETHERNET_RAW_LINE is not defined in variants/rak4631/platformio.ini.
• FORCE_CLIENT_REPEAT is present in both examples/companion_radio/MyMesh.cpp and variants/rak4631/platformio.ini.
• Both canonical RAK13800 Companion targets include helpers/nrf52/SerialEthernetInterface.cpp.
• Both canonical RAK13800 Companion targets include the RAKWireless/RAK13800-W5100S dependency.

The standout functional change is multi-client framed Companion TCP: multiple Companion clients can stay connected at the same time, push frames can be broadcast, and command responses route back to the client that sent the command.