Add stateless lambda infrastructure to TemplatableValue, Action, and Condition

Add STATELESS_LAMBDA support to TemplatableValue class using C++20 concepts:
- Automatically detects stateless lambdas (convertible to function pointers)
- Uses 4-byte function pointer instead of 32-byte std::function
- Falls back to std::function for stateful lambdas

Add StatelessLambdaAction and StatelessLambdaCondition classes for automation:
- Memory: 4 bytes vs 32 bytes per lambda
- Direct function pointer calls (no std::function overhead)

This infrastructure enables template platforms to use function pointers for
stateless lambdas, saving ~28 bytes per lambda instance.

esphome/components/esp32_ble_server/ble_server_automations.cpp

@@ -15,7 +15,10 @@ Trigger<std::vector<uint8_t>, uint16_t> *BLETriggers::create_characteristic_on_w
   Trigger<std::vector<uint8_t>, uint16_t> *on_write_trigger =  // NOLINT(cppcoreguidelines-owning-memory)
       new Trigger<std::vector<uint8_t>, uint16_t>();
   characteristic->on_write([on_write_trigger](std::span<const uint8_t> data, uint16_t id) {
-    // Convert span to vector for trigger
+    // Convert span to vector for trigger - copy is necessary because:
+    // 1. Trigger stores the data for use in automation actions that execute later
+    // 2. The span is only valid during this callback (points to temporary BLE stack data)
+    // 3. User lambdas in automations need persistent data they can access asynchronously
     on_write_trigger->trigger(std::vector<uint8_t>(data.begin(), data.end()), id);
   });
   return on_write_trigger;
@@ -27,7 +30,10 @@ Trigger<std::vector<uint8_t>, uint16_t> *BLETriggers::create_descriptor_on_write
   Trigger<std::vector<uint8_t>, uint16_t> *on_write_trigger =  // NOLINT(cppcoreguidelines-owning-memory)
       new Trigger<std::vector<uint8_t>, uint16_t>();
   descriptor->on_write([on_write_trigger](std::span<const uint8_t> data, uint16_t id) {
-    // Convert span to vector for trigger
+    // Convert span to vector for trigger - copy is necessary because:
+    // 1. Trigger stores the data for use in automation actions that execute later
+    // 2. The span is only valid during this callback (points to temporary BLE stack data)
+    // 3. User lambdas in automations need persistent data they can access asynchronously
     on_write_trigger->trigger(std::vector<uint8_t>(data.begin(), data.end()), id);
   });
   return on_write_trigger;

esphome/core/automation.h

@@ -27,11 +27,20 @@ template<typename T, typename... X> class TemplatableValue {
  public:
   TemplatableValue() : type_(NONE) {}
 
-  template<typename F, enable_if_t<!is_invocable<F, X...>::value, int> = 0> TemplatableValue(F value) : type_(VALUE) {
+  template<typename F> TemplatableValue(F value) requires(!std::invocable<F, X...>) : type_(VALUE) {
     new (&this->value_) T(std::move(value));
   }
 
-  template<typename F, enable_if_t<is_invocable<F, X...>::value, int> = 0> TemplatableValue(F f) : type_(LAMBDA) {
+  // For stateless lambdas (convertible to function pointer): use function pointer
+  template<typename F>
+  TemplatableValue(F f) requires std::invocable<F, X...> && std::convertible_to<F, T (*)(X...)>
+      : type_(STATELESS_LAMBDA) {
+    this->stateless_f_ = f;  // Implicit conversion to function pointer
+  }
+
+  // For stateful lambdas (not convertible to function pointer): use std::function
+  template<typename F>
+  TemplatableValue(F f) requires std::invocable<F, X...> &&(!std::convertible_to<F, T (*)(X...)>) : type_(LAMBDA) {
     this->f_ = new std::function<T(X...)>(std::move(f));
   }
 
@@ -41,6 +50,8 @@ template<typename T, typename... X> class TemplatableValue {
       new (&this->value_) T(other.value_);
     } else if (type_ == LAMBDA) {
       this->f_ = new std::function<T(X...)>(*other.f_);
+    } else if (type_ == STATELESS_LAMBDA) {
+      this->stateless_f_ = other.stateless_f_;
     }
   }
 
@@ -51,6 +62,8 @@ template<typename T, typename... X> class TemplatableValue {
     } else if (type_ == LAMBDA) {
       this->f_ = other.f_;
       other.f_ = nullptr;
+    } else if (type_ == STATELESS_LAMBDA) {
+      this->stateless_f_ = other.stateless_f_;
     }
     other.type_ = NONE;
   }
@@ -78,16 +91,23 @@ template<typename T, typename... X> class TemplatableValue {
     } else if (type_ == LAMBDA) {
       delete this->f_;
     }
+    // STATELESS_LAMBDA/NONE: no cleanup needed (function pointer or empty, not heap-allocated)
   }
 
   bool has_value() { return this->type_ != NONE; }
 
   T value(X... x) {
-    if (this->type_ == LAMBDA) {
-      return (*this->f_)(x...);
+    switch (this->type_) {
+      case STATELESS_LAMBDA:
+        return this->stateless_f_(x...);  // Direct function pointer call
+      case LAMBDA:
+        return (*this->f_)(x...);  // std::function call
+      case VALUE:
+        return this->value_;
+      case NONE:
+      default:
+        return T{};
     }
-    // return value also when none
-    return this->type_ == VALUE ? this->value_ : T{};
   }
 
   optional<T> optional_value(X... x) {
@@ -109,11 +129,13 @@ template<typename T, typename... X> class TemplatableValue {
     NONE,
     VALUE,
     LAMBDA,
+    STATELESS_LAMBDA,
   } type_;
 
   union {
     T value_;
     std::function<T(X...)> *f_;
+    T (*stateless_f_)(X...);
   };
 };
 

esphome/core/base_automation.h

@@ -79,6 +79,18 @@ template<typename... Ts> class LambdaCondition : public Condition<Ts...> {
   std::function<bool(Ts...)> f_;
 };
 
+/// Optimized lambda condition for stateless lambdas (no capture).
+/// Uses function pointer instead of std::function to reduce memory overhead.
+/// Memory: 4 bytes (function pointer on 32-bit) vs 32 bytes (std::function).
+template<typename... Ts> class StatelessLambdaCondition : public Condition<Ts...> {
+ public:
+  explicit StatelessLambdaCondition(bool (*f)(Ts...)) : f_(f) {}
+  bool check(Ts... x) override { return this->f_(x...); }
+
+ protected:
+  bool (*f_)(Ts...);
+};
+
 template<typename... Ts> class ForCondition : public Condition<Ts...>, public Component {
  public:
   explicit ForCondition(Condition<> *condition) : condition_(condition) {}
@@ -190,6 +202,19 @@ template<typename... Ts> class LambdaAction : public Action<Ts...> {
   std::function<void(Ts...)> f_;
 };
 
+/// Optimized lambda action for stateless lambdas (no capture).
+/// Uses function pointer instead of std::function to reduce memory overhead.
+/// Memory: 4 bytes (function pointer on 32-bit) vs 32 bytes (std::function).
+template<typename... Ts> class StatelessLambdaAction : public Action<Ts...> {
+ public:
+  explicit StatelessLambdaAction(void (*f)(Ts...)) : f_(f) {}
+
+  void play(Ts... x) override { this->f_(x...); }
+
+ protected:
+  void (*f_)(Ts...);
+};
+
 template<typename... Ts> class IfAction : public Action<Ts...> {
  public:
   explicit IfAction(Condition<Ts...> *condition) : condition_(condition) {}