fix(clients/rust): use ucobs instead of cobs crate

Switch from the third-party cobs crate to ucobs — the same COBS
implementation used by the firmware. Keeps the entire stack on one
COBS library.

clients/rust/Cargo.toml

@@ -11,7 +11,7 @@ categories = ["hardware-support"]
 
 [dependencies]
 anyhow = "1"
-cobs = "0.3"
+ucobs = "0.3"
 serialport = "4"
 tracing = "0.1"