fix: exclusive serial port lock prevents USB port theft

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.

Cargo.lock

@@ -161,9 +161,9 @@ checksum = "773648b94d0e5d620f64f280777445740e61fe701025087ec8b57f45c791888b"
 
 [[package]]
 name = "donglora-client"
-version = "0.2.0"
+version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1936935d31ab56a8cd70ceecfd6d447f2628a73a123e1cfe37ee4987eab0fa70"
+checksum = "343d750fe1e5dad94d4b31e6dbbb05214b39e66f3c94a2ff2a29b0031ffd2826"
 dependencies = [
  "anyhow",
  "serialport",
@@ -173,7 +173,7 @@ dependencies = [
 
 [[package]]
 name = "donglora-mux"
-version = "0.2.0"
+version = "0.2.1"
 dependencies = [
  "anyhow",
  "clap",