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/README.md

@@ -31,7 +31,7 @@ loop {
 
 ## Dependencies
 
-- `cobs` — COBS framing
+- `ucobs` — COBS framing (same implementation as the firmware)
 - `serialport` — USB serial communication
 - `anyhow` — error handling
 - `tracing` — logging