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/esp_now_comm.h

@@ -2,8 +2,15 @@
 #define ESP_NOW_COMM_H
 
 #include "app_config.h"
+#include "client_registry.h"
 #include "esp_err.h"
 
 esp_err_t esp_now_comm_init(const app_config_t *config);
 
+/**
+ * Master: broadcast accel deadzone (same path as DISCOVER).
+ * client_id 0 = all slaves; otherwise only the slave with that id applies it.
+ */
+esp_err_t esp_now_comm_send_accel_deadzone(uint32_t client_id, uint32_t deadzone);
+
 #endif