refactor: deduplicate USB/UART protocol framing

- Made protocol_io.rs compile for all boards (not just heltec_v3)
- Renamed FrameAccumulator → CobsDecoder, encode_response → cobs_encode_response
- Refactored USB task to use shared CobsDecoder and route_command from protocol_io
- Removed ~80 lines of duplicated COBS framing and command routing from usb/task.rs
- Single canonical MAX_FRAME definition in protocol_io.rs

src/main.rs

@@ -24,7 +24,7 @@ mod display;
 mod protocol;
 #[cfg(not(test))]
 mod radio;
-#[cfg(all(not(test), feature = "heltec_v3"))]
+#[cfg(not(test))]
 mod protocol_io;
 #[cfg(all(not(test), feature = "heltec_v3"))]
 mod uart;