Add accelerometer deadzone via UART and ESP-NOW.

Filter BMA456 logs by configurable LSB threshold; master can set deadzone
for local sensor or slaves using ACCEL_DEADZONE (UART) and ESP-NOW broadcast
until unicast delivery is restored.

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

main/proto/esp_now_messages.proto

@@ -7,6 +7,7 @@ enum EspNowMessageType {
   ESPNOW_DISCOVER = 1;
   ESPNOW_SLAVE_INFO = 2;
   ESPNOW_HEARTBEAT = 3;
+  ESPNOW_SET_ACCEL_DEADZONE = 4;
 }
 
 message EspNowDiscover {
@@ -22,11 +23,17 @@ message EspNowSlavePresence {
   bool used = 6;
 }
 
+message EspNowAccelDeadzone {
+  uint32 deadzone = 1;
+  uint32 client_id = 2; // 0 = all slaves; otherwise only matching slave_id applies
+}
+
 message EspNowMessage {
   EspNowMessageType type = 1;
   oneof payload {
     EspNowDiscover discover = 2;
     EspNowSlavePresence slave_info = 3;
     EspNowSlavePresence heartbeat = 4;
+    EspNowAccelDeadzone accel_deadzone = 5;
   }
 }