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/client_registry.h

@@ -19,8 +19,12 @@ typedef struct {
   /** Milliseconds since boot when last heartbeat / SLAVE_INFO was accepted. */
   uint32_t last_success_ping_at;
   uint32_t version;
+  /** Accel deadzone in raw LSB per axis (master copy for ESP-NOW config). */
+  uint32_t accel_deadzone;
 } client_info_t;
 
+#define CLIENT_REGISTRY_DEFAULT_ACCEL_DEADZONE 100u
+
 void client_registry_init(void);
 
 /** Milliseconds since boot (same clock as stored ping timestamps). */
@@ -49,5 +53,14 @@ void client_registry_check_timeouts(uint32_t timeout_ms);
 size_t client_registry_count(void);
 const client_info_t *client_registry_at(size_t index);
 const client_info_t *client_registry_find_by_mac(const uint8_t mac[CLIENT_MAC_LEN]);
+const client_info_t *client_registry_find_by_id(uint32_t id);
+
+esp_err_t client_registry_set_accel_deadzone(uint32_t client_id,
+                                             uint32_t deadzone);
+esp_err_t client_registry_get_accel_deadzone(uint32_t client_id,
+                                             uint32_t *deadzone_out);
+
+/** Push deadzone to all active registry entries; returns count updated. */
+size_t client_registry_set_accel_deadzone_all(uint32_t deadzone);
 
 #endif