Serial Bus #1: Add Serial Bus Module to handle multiple ESPs

[JensOgorek]

This PR adds a SerialBus module so multiple ESP32-based Lizard nodes can share one UART link with a single coordinator that orchestrates traffic round-robin style.

Commands:

• bus.send(receiver, payload) – enqueue a point-to-point frame (payload must fit in 256 bytes, no newlines).
• bus.make_coordinator(peer_ids...) – marks this node as coordinator and loads the peer poll order.

How it works:
Each instance runs a background FreeRTOS task that reads $$sender:receiver$$payload frames from UART, parses control words (__POLL__, __DONE__, __BUS_RESPONSE__), and either executes remote commands or enqueues outbound traffic. Coordinators open transmit windows for their peers in turn and enforce timeouts; non-coordinators only transmit while polled, then reply with __DONE__ once their queue is flushed.