Add local WebSocket relay server and ESP32 ultrasonic firmware example

[musfaagv]

Motivation

• Provide a simple local WebSocket relay so an ESP32 firmware example can send sensor data to a web UI and receive commands over the local network.
• Standardize a compact JSON payload format for ultrasonic sensor data and LED commands for easier integration and testing.
• Offer a ready-to-run Python server and a matching ESP32 sketch to help users prototype without needing external infrastructure.

Description

• Add example_websocket_server/server.py implementing a relay server (listens on ws://0.0.0.0:8765) with a connected-client registry, broadcast to other clients, payload parsing/validation, and structured error responses for invalid JSON or unknown message types.
• Add example_websocket_server/requirements.txt (websockets>=14.0,<16.0) and example_websocket_server/README.md with setup and run instructions (e.g. python -m venv .venv, install deps, python server.py).
• Add ESP32 firmware sample example_firmware/esp32_ws_ultrasonic/esp32_ws_ultrasonic.ino that connects to configurable Wi‑Fi and ws://<IP_LAPTOP>:8765, reads HC-SR04 distance (cm/inch), sends periodic JSON sensor payloads, and processes {"type":"led","state":"on"} / {"type":"led","state":"off"} commands to toggle a GPIO LED.
• Update example_firmware/README.md with configuration steps (SSID/password, laptop IP), pin mapping, payload examples, dependency notes, and server run steps.

Testing

• Ran python -m py_compile example_websocket_server/server.py which completed successfully.

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Codex Task