Add comment explaining early guard

esphome/components/ld2412/ld2412.cpp

@@ -310,6 +310,8 @@ void LD2412Component::restart_and_read_all_info() {
 }
 
 void LD2412Component::loop() {
+  // Early return avoids stack adjustment for the batch buffer below.
+  // loop() runs ~7000/min so most calls have nothing to read.
   int avail = this->available();
   if (avail <= 0) {
     return;