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/bosch456.h

@@ -9,7 +9,17 @@
 
 #define BMA456_ADDRESS 0x18
 
+#define BMA456_DEFAULT_ACCEL_DEADZONE 100u
+
 /** Initialize BMA456 on the shared I2C bus. Returns ESP_OK or logs and skips sensor use. */
 esp_err_t init_bma456(i2c_master_bus_handle_t bus_handle);
 
+bool bma456_is_ready(void);
+
+void bma456_set_accel_deadzone(uint32_t deadzone_lsb);
+uint32_t bma456_get_accel_deadzone(void);
+
+/** Log accel sample only when any axis changed more than deadzone since last report. */
+void bma456_report_accel_if_changed(int16_t x, int16_t y, int16_t z);
+
 #endif