Add uart_cmd helpers to deduplicate UART command handlers.

Centralize protobuf decode, response init/send, registration, and common
nanopb encode callbacks; refactor existing cmd_* modules to use them.

Co-authored-by: Cursor <cursoragent@cursor.com>

main/uart_cmd.h

@@ -0,0 +1,37 @@
+#ifndef UART_CMD_H
+#define UART_CMD_H
+
+#include "cmd_handler.h"
+#include "esp_err.h"
+#include "pb.h"
+#include "uart_messages.pb.h"
+
+/** Decode framed UART body (protobuf UartMessage without the leading type byte). */
+esp_err_t uart_cmd_decode(const uint8_t *data, size_t len, alox_UartMessage *out);
+
+/** Set type + oneof tag on an outbound UartMessage. */
+void uart_cmd_init_response(alox_UartMessage *msg, alox_MessageType type,
+                            pb_size_t which_payload);
+
+/** Encode and send; logs on failure when log_tag is non-NULL. */
+esp_err_t uart_cmd_send(const alox_UartMessage *msg, const char *log_tag);
+
+esp_err_t uart_cmd_register(alox_MessageType type, msg_callback_t cb);
+
+/** Nanopb encode callback for a NUL-terminated C string. */
+bool uart_cmd_encode_string(pb_ostream_t *stream, const pb_field_t *field,
+                            void *const *arg);
+
+typedef struct {
+  const uint8_t *data;
+  size_t len;
+} uart_cmd_bytes_t;
+
+/** Nanopb encode callback; arg is uart_cmd_bytes_t *. */
+bool uart_cmd_encode_bytes(pb_ostream_t *stream, const pb_field_t *field,
+                           void *const *arg);
+
+#define UART_CMD_REQ(msg, tag, field)                                          \
+  ((msg)->which_payload == (tag) ? &(msg)->payload.field : NULL)
+
+#endif