[light] Replace initial_state storage with flash-resident callback

Replace the inline optional<LightStateRTCState> (~44 bytes) with a
function pointer callback (4 bytes) that populates the initial state
during setup. The callback is a stateless lambda whose values live in
flash as code, not in the LightState object.

Saves ~40 bytes per LightState instance unconditionally — the optional
reserved space for the struct even when initial_state was not configured
(the common case).

esphome/components/light/__init__.py

@@ -38,7 +38,7 @@ from esphome.const import (
     CONF_WEB_SERVER,
     CONF_WHITE,
 )
-from esphome.core import CORE, ID, CoroPriority, HexInt, coroutine_with_priority
+from esphome.core import CORE, ID, CoroPriority, HexInt, Lambda, coroutine_with_priority
 from esphome.core.entity_helpers import entity_duplicate_validator, setup_entity
 from esphome.cpp_generator import MockObjClass
 
@@ -52,6 +52,7 @@ from .effects import (
     validate_effects,
 )
 from .types import (  # noqa
+    COLOR_MODES,
     AddressableLight,
     AddressableLightState,
     ColorMode,
@@ -262,8 +263,12 @@ async def setup_light_core_(light_var, config, output_var):
     cg.add(light_var.set_restore_mode(config[CONF_RESTORE_MODE]))
 
     if (initial_state_config := config.get(CONF_INITIAL_STATE)) is not None:
+        # Emit a stateless lambda that constructs the initial state — values live
+        # in flash as code, not stored in the LightState object (~40 bytes saved).
+        color_mode_str = initial_state_config.get(CONF_COLOR_MODE)
+        color_mode_expr = COLOR_MODES.get(color_mode_str, ColorMode.UNKNOWN)
         initial_state = LightStateRTCState(
-            initial_state_config.get(CONF_COLOR_MODE, ColorMode.UNKNOWN),
+            color_mode_expr,
             initial_state_config.get(CONF_STATE, False),
             initial_state_config.get(CONF_BRIGHTNESS, 1.0),
             initial_state_config.get(CONF_COLOR_BRIGHTNESS, 1.0),
@@ -275,7 +280,13 @@ async def setup_light_core_(light_var, config, output_var):
             initial_state_config.get(CONF_COLD_WHITE, 1.0),
             initial_state_config.get(CONF_WARM_WHITE, 1.0),
         )
-        cg.add(light_var.set_initial_state(initial_state))
+        args = [(LightStateRTCState.operator("ref"), "s")]
+        lamb = await cg.process_lambda(
+            Lambda(f"s = {initial_state};"),
+            args,
+            return_type=cg.void,
+        )
+        cg.add(light_var.set_initial_state(lamb))
 
     if (
         default_transition_length := config.get(CONF_DEFAULT_TRANSITION_LENGTH)

esphome/components/light/light_state.cpp

@@ -37,8 +37,9 @@ void LightState::setup() {
 
   auto call = this->make_call();
   LightStateRTCState recovered{};
-  if (this->initial_state_.has_value()) {
-    recovered = *this->initial_state_;
+  if (this->initial_state_callback_) {
+    this->initial_state_callback_(recovered);
+    this->initial_state_callback_ = nullptr;  // One-shot — no longer needed
   }
   switch (this->restore_mode_) {
     case LIGHT_RESTORE_DEFAULT_OFF:
@@ -195,7 +196,7 @@ void LightState::set_flash_transition_length(uint32_t flash_transition_length) {
 uint32_t LightState::get_flash_transition_length() const { return this->flash_transition_length_; }
 void LightState::set_gamma_correct(float gamma_correct) { this->gamma_correct_ = gamma_correct; }
 void LightState::set_restore_mode(LightRestoreMode restore_mode) { this->restore_mode_ = restore_mode; }
-void LightState::set_initial_state(const LightStateRTCState &initial_state) { this->initial_state_ = initial_state; }
+void LightState::set_initial_state(void (*callback)(LightStateRTCState &)) { this->initial_state_callback_ = callback; }
 bool LightState::supports_effects() { return !this->effects_.empty(); }
 const FixedVector<LightEffect *> &LightState::get_effects() const { return this->effects_; }
 void LightState::add_effects(const std::initializer_list<LightEffect *> &effects) {

esphome/components/light/light_state.h

@@ -188,8 +188,9 @@ class LightState : public EntityBase, public Component {
   /// Set the restore mode of this light
   void set_restore_mode(LightRestoreMode restore_mode);
 
-  /// Set the initial state of this light
-  void set_initial_state(const LightStateRTCState &initial_state);
+  /// Set a callback to populate the initial state defaults during setup.
+  /// The callback is called once, then cleared. Values live in flash as code.
+  void set_initial_state(void (*callback)(LightStateRTCState &));
 
   /// Return whether the light has any effects that meet the trait requirements.
   bool supports_effects();
@@ -342,8 +343,9 @@ class LightState : public EntityBase, public Component {
    */
   std::unique_ptr<std::vector<LightTargetStateReachedListener *>> target_state_reached_listeners_;
 
-  /// Initial state of the light.
-  optional<LightStateRTCState> initial_state_{};
+  /// Callback to populate initial state defaults — called once during setup, then cleared.
+  /// Values live in flash as function body; no per-instance data storage beyond this pointer.
+  void (*initial_state_callback_)(LightStateRTCState &){nullptr};
 
   /// Value for storing the index of the currently active effect. 0 if no effect is active
   uint32_t active_effect_index_{};