Fix ESP-NOW unicast by using sender MAC in client registry.

Register slaves from recv src_addr instead of protobuf mac bytes, add
ESPNOW_UNICAST_TEST for path verification, restore unicast deadzone, and
expose unicast-test in goTool.

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

main/esp_now_comm.h

@@ -7,10 +7,12 @@
 
 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);
+/** Master: unicast accel deadzone to one slave (client_id is echoed for filtering). */
+esp_err_t esp_now_comm_send_accel_deadzone(const uint8_t mac[CLIENT_MAC_LEN],
+                                           uint32_t client_id, uint32_t deadzone);
+
+/** Master: send minimal unicast test payload to verify master→slave path. */
+esp_err_t esp_now_comm_send_unicast_test(const uint8_t mac[CLIENT_MAC_LEN],
+                                         uint32_t seq);
 
 #endif