[scheduler] Reduce lock acquisitions in process_defer_queue_ (#14107)

esphome/core/scheduler.cpp

@@ -421,6 +421,29 @@ void Scheduler::full_cleanup_removed_items_() {
   this->to_remove_ = 0;
 }
 
+#ifndef ESPHOME_THREAD_SINGLE
+void Scheduler::compact_defer_queue_locked_() {
+  // Rare case: new items were added during processing - compact the vector
+  // This only happens when:
+  // 1. A deferred callback calls defer() again, or
+  // 2. Another thread calls defer() while we're processing
+  //
+  // Move unprocessed items (added during this loop) to the front for next iteration
+  //
+  // SAFETY: Compacted items may include cancelled items (marked for removal via
+  // cancel_item_locked_() during execution). This is safe because should_skip_item_()
+  // checks is_item_removed_() before executing, so cancelled items will be skipped
+  // and recycled on the next loop iteration.
+  size_t remaining = this->defer_queue_.size() - this->defer_queue_front_;
+  for (size_t i = 0; i < remaining; i++) {
+    this->defer_queue_[i] = std::move(this->defer_queue_[this->defer_queue_front_ + i]);
+  }
+  // Use erase() instead of resize() to avoid instantiating _M_default_append
+  // (saves ~156 bytes flash). Erasing from the end is O(1) - no shifting needed.
+  this->defer_queue_.erase(this->defer_queue_.begin() + remaining, this->defer_queue_.end());
+}
+#endif /* not ESPHOME_THREAD_SINGLE */
+
 void HOT Scheduler::call(uint32_t now) {
 #ifndef ESPHOME_THREAD_SINGLE
   this->process_defer_queue_(now);