Add UART ACCEL_READ command for on-demand BMA456 samples.

Expose MessageType 24 with protobuf response (success, x, y, z in raw LSB),
firmware handler with mutex-safe I2C read, goTool `accel` CLI, and docs.

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

main/cmd/cmd_accel_read.c

@@ -0,0 +1,34 @@
+#include "bosch456.h"
+#include "cmd_accel_read.h"
+#include "uart_cmd.h"
+
+static const char *TAG = "[ACCEL_READ]";
+
+static void reply(bool success, int16_t x, int16_t y, int16_t z) {
+  alox_UartMessage response;
+  uart_cmd_init_response(&response, alox_MessageType_ACCEL_READ,
+                         alox_UartMessage_accel_read_response_tag);
+  response.payload.accel_read_response.success = success;
+  response.payload.accel_read_response.x = x;
+  response.payload.accel_read_response.y = y;
+  response.payload.accel_read_response.z = z;
+  uart_cmd_send(&response, TAG);
+}
+
+static void handle_accel_read(const uint8_t *data, size_t len) {
+  (void)data;
+  (void)len;
+
+  int16_t x = 0;
+  int16_t y = 0;
+  int16_t z = 0;
+  if (bma456_read_accel(&x, &y, &z) == ESP_OK) {
+    reply(true, x, y, z);
+    return;
+  }
+  reply(false, 0, 0, 0);
+}
+
+void cmd_accel_read_register(void) {
+  uart_cmd_register(alox_MessageType_ACCEL_READ, handle_accel_read);
+}

main/cmd/cmd_accel_read.h

@@ -0,0 +1,6 @@
+#ifndef CMD_ACCEL_READ_H
+#define CMD_ACCEL_READ_H
+
+void cmd_accel_read_register(void);
+
+#endif

main/cmd/cmd_handler.c

@@ -48,6 +48,8 @@ static const char *message_type_name(uint16_t id) {
     return "FIND_ME";
   case alox_MessageType_RESTART:
     return "RESTART";
+  case alox_MessageType_ACCEL_READ:
+    return "ACCEL_READ";
   default:
     return "UNKNOWN";
   }