[core] Optimize looping_components_ with FixedVector to save flash

esphome/core/application.cpp

@@ -340,8 +340,8 @@ void Application::calculate_looping_components_() {
     }
   }
 
-  // Pre-reserve vector to avoid reallocations
-  this->looping_components_.reserve(total_looping);
+  // Initialize FixedVector with exact size - no reallocation possible
+  this->looping_components_.init(total_looping);
 
   // Add all components with loop override that aren't already LOOP_DONE
   // Some components (like logger) may call disable_loop() during initialization

esphome/core/application.h

@@ -472,7 +472,7 @@ class Application {
   // - When a component is enabled, it's swapped with the first inactive component
   //   and active_end_ is incremented
   // - This eliminates branch mispredictions from flag checking in the hot loop
-  std::vector<Component *> looping_components_{};
+  FixedVector<Component *> looping_components_{};
 #ifdef USE_SOCKET_SELECT_SUPPORT
   std::vector<int> socket_fds_;  // Vector of all monitored socket file descriptors
 #endif

esphome/core/helpers.h

@@ -159,6 +159,50 @@ template<typename T, size_t N> class StaticVector {
   const_reverse_iterator rend() const { return const_reverse_iterator(begin()); }
 };
 
+/// Fixed-capacity vector - allocates once at runtime, never reallocates
+/// This avoids std::vector template overhead (_M_realloc_insert, _M_default_append)
+/// when size is known at initialization but not at compile time
+template<typename T> class FixedVector {
+ private:
+  T *data_{nullptr};
+  size_t size_{0};
+  size_t capacity_{0};
+
+ public:
+  FixedVector() = default;
+
+  ~FixedVector() {
+    if (data_ != nullptr) {
+      delete[] data_;
+    }
+  }
+
+  // Disable copy to avoid accidental copies
+  FixedVector(const FixedVector &) = delete;
+  FixedVector &operator=(const FixedVector &) = delete;
+
+  // Allocate capacity - can only be called once on empty vector
+  void init(size_t n) {
+    if (data_ == nullptr && n > 0) {
+      data_ = new T[n];
+      capacity_ = n;
+      size_ = 0;
+    }
+  }
+
+  // Add element (no reallocation - must have initialized capacity)
+  void push_back(const T &value) {
+    if (size_ < capacity_) {
+      data_[size_++] = value;
+    }
+  }
+
+  size_t size() const { return size_; }
+
+  T &operator[](size_t i) { return data_[i]; }
+  const T &operator[](size_t i) const { return data_[i]; }
+};
+
 ///@}
 
 /// @name Mathematics