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/proto/esp_now_messages.proto

@@ -8,6 +8,11 @@ enum EspNowMessageType {
   ESPNOW_SLAVE_INFO = 2;
   ESPNOW_HEARTBEAT = 3;
   ESPNOW_SET_ACCEL_DEADZONE = 4;
+  ESPNOW_UNICAST_TEST = 5;
+}
+
+message EspNowUnicastTest {
+  uint32 seq = 1;
 }
 
 message EspNowDiscover {
@@ -35,5 +40,6 @@ message EspNowMessage {
     EspNowSlavePresence slave_info = 3;
     EspNowSlavePresence heartbeat = 4;
     EspNowAccelDeadzone accel_deadzone = 5;
+    EspNowUnicastTest unicast_test = 6;
   }
 }