fix: sticky mux reconnect prevents USB port theft

Once the bridge connects via mux, all subsequent reconnects use mux-only
mode (connect_mux_auto). Previously, each reconnect re-ran the full
mux→USB fallback chain, which could steal the serial port from the mux
during a brief disconnect.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "donglora-bridge"
-version = "0.3.0"
+version = "0.3.1"
 edition = "2024"
 authors = ["Stephen Waits <steve@waits.net>"]
 license = "MIT"