[1sthandy]

Split out of #2679 (which bundled it with PoE support, now #3827). Addresses the same problem as #2972, which targets the pre-refactor file layout.

Problem

RAK13800EthernetInterface holds a single EthernetClient and stops the live connection whenever another client connects. With Home Assistant and the phone app both pointed at the same node, they kick each other off the one socket and reconnect in a loop.

Change

Keep up to MAX_ETH_CLIENTS (default 3; the W5100S has 4 sockets and one is the listener) connections open at once:

• per-client connection slot and receive-parser state, with round-robin polling so one busy client cannot starve the others
• command responses go only to the client that sent the last command; push frames (code >= 0x80, such as adverts or LoRa-RX logs) are broadcast to all clients
• a new client is rejected only when every slot is busy, and a dropped client frees just its own slot

Framing is unchanged (< / > plus a uint16 LE length), and a single client behaves as before.

Scope

Only RAK13800EthernetInterface.{h,cpp} changes. checkRecvFrame() and writeFrame() are overridden there because routing to several clients needs per-client state that the shared single-client SerialEthernetInterface does not track; the shared base, MultiSerialInterface, EthernetInterface.h and the CH390 driver are untouched. available() / read() / write() are kept as trivial stubs only to satisfy the base class's pure-virtual contract. If you would rather have multi-client support in the shared base so CH390 benefits too, I am happy to rework it that way.

Testing

• Builds: RAK_4631_companion_radio_ethernet, RAK_4631_repeater_ethernet.
• Hardware (RAK10720): Home Assistant and the phone app connected to the same node at the same time and stayed connected.

This and #3827 both touch RAK13800EthernetInterface.{h,cpp}, so whichever is merged second will need a small rebase.

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