feat: add SerialOpener hook to FlasherOptions

[jgangemi]

Summary

Adds an optional SerialOpener hook to FlasherOptions so callers can intercept how serial ports are opened, enabling coordinated multi-consumer access to a single physical port.

Motivation

When a tool multiplexes one serial port between a live monitor and the flasher, having the flasher call serial.Open directly conflicts with the externally-managed port handle. This hook lets the caller supply its own opener — e.g. one that coordinates with a running monitor — used for both the initial open and the reopen-after-USB-reenumeration path.

Details

• New field SerialOpener func(name string, mode *serial.Mode) (serial.Port, error) on FlasherOptions.
• New() and reopenPort() route opens through a small opts.openPort() helper that uses SerialOpener when set and falls back to serial.Open otherwise.
• Fully backward compatible: a nil hook preserves current behavior exactly.
• Includes a unit test verifying the hook is invoked.

[deadprogram]

cc @georgik

[deadprogram]

Thank you for the addition here @jgangemi now merging.