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/proto/uart_messages.proto

@@ -22,6 +22,7 @@ enum MessageType {
   OTA_SLAVE_PROGRESS = 21;
   FIND_ME = 22;
   RESTART = 23;
+  ACCEL_READ = 24;
 }
 
 message UartMessage {
@@ -48,6 +49,8 @@ message UartMessage {
     EspNowFindMeResponse espnow_find_me_response = 20;
     RestartRequest restart_request = 21;
     RestartResponse restart_response = 22;
+    AccelReadRequest accel_read_request = 23;
+    AccelReadResponse accel_read_response = 24;
   }
 }
 
@@ -106,6 +109,16 @@ message AccelDeadzoneResponse {
   uint32 slaves_updated = 4;
 }
 
+// Host → device: read current BMA456 accelerometer sample (raw LSB, ±2g range).
+message AccelReadRequest {}
+
+message AccelReadResponse {
+  bool success = 1;
+  sint32 x = 2;
+  sint32 y = 3;
+  sint32 z = 4;
+}
+
 message EspNowUnicastTestRequest {
   uint32 client_id = 1;
   uint32 seq = 2;