UART boards (CP2102 bridge) keep the serial port alive across ESP32 resets,
so the mux never detected the board rebooting. Now the writer sends a
keepalive Ping every 5s of idle and declares the dongle lost after a
2s timeout.
On reconnect, saved radio config and RX state are re-sent to the dongle
so connected clients continue working without re-establishing state.
The mux now acquires an flock-based exclusive lock on the serial port
before opening it. This prevents bridges or other processes from directly
opening the same device while the mux owns it. The lock is held for the
duration of the dongle session and auto-released on disconnect or crash.
swaits