WebSerial port ownership was previously held inside StepFlash, which
made it awkward for other steps (StepVerify) to read from the same
port without fighting over exclusive access. useSerialConnection
centralizes port open/close/read/write so the wizard's steps share one
connection lifecycle instead of each managing their own.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>