Add serial port support with SerialDevice and SerialConnection

Objo should provide built-in support for communicating with serial ports and USB devices that expose themselves as serial ports. This would unblock common hobbyist and hardware workflows such as Arduino boards, USB-to-serial adapters, barcode scanners, scales, CNC controllers, lab equipment, and other devices that communicate over COM ports or /dev/tty* / /dev/cu.* paths.

A first version should expose discoverable SerialDevice objects and a SerialConnection class for opening a port, configuring baud rate and framing, reading and writing text or bytes, handling line endings, and reacting to data, line-state changes, errors, and closure.