fix: connect() no longer falls through from mux to USB serial

When a mux socket file exists, connect(None, timeout) and try_connect()
now commit to the mux — returning an error if unreachable instead of
silently falling through to USB serial. This prevents a race condition
where a client could steal the serial port from the mux during reconnect.

Callers with explicit port = Some(...) are unaffected.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "donglora-client"
-version = "0.2.0"
+version = "0.2.1"
 edition = "2024"
 rust-version = "1.85"
 authors = ["Stephen Waits <steve@waits.net>"]