[light] Use compact per-field union storage in LightControlAction

Replace 14 TemplatableValue fields with a per-field union that stores
either a constant value or a callable pointer in the same 4 bytes
(on 32-bit targets). A 2-bit-per-field type tag in a uint32_t tracks
field state (unset/constant/stateless lambda/stateful lambda).

Reduces LightControlAction from 128 to 76 bytes per instance on ESP32.
For a config with 8 light actions, this saves 416 bytes of RAM and
476 bytes of flash (from eliminated TemplatableValue template
instantiations).

tests/integration/fixtures/light_control_action.yaml

@@ -0,0 +1,139 @@
+esphome:
+  name: light-control-action-test
+host:
+api:  # Port will be automatically injected
+logger:
+  level: DEBUG
+
+globals:
+  - id: test_brightness
+    type: float
+    initial_value: "0.75"
+
+output:
+  - platform: template
+    id: test_red
+    type: float
+    write_action:
+      - lambda: ""
+  - platform: template
+    id: test_green
+    type: float
+    write_action:
+      - lambda: ""
+  - platform: template
+    id: test_blue
+    type: float
+    write_action:
+      - lambda: ""
+  - platform: template
+    id: test_cold_white
+    type: float
+    write_action:
+      - lambda: ""
+  - platform: template
+    id: test_warm_white
+    type: float
+    write_action:
+      - lambda: ""
+
+light:
+  - platform: rgbww
+    name: "Test Light"
+    id: test_light
+    red: test_red
+    green: test_green
+    blue: test_blue
+    cold_white: test_cold_white
+    warm_white: test_warm_white
+    cold_white_color_temperature: 6536 K
+    warm_white_color_temperature: 2000 K
+    effects:
+      - random:
+          name: "Test Effect"
+          transition_length: 10ms
+          update_interval: 10ms
+
+button:
+  # Test 1: light.turn_on with RGB constants
+  - platform: template
+    id: btn_turn_on_rgb
+    name: "Turn On RGB"
+    on_press:
+      - light.turn_on:
+          id: test_light
+          brightness: 1.0
+          red: 0.0
+          green: 0.0
+          blue: 1.0
+
+  # Test 2: light.turn_off
+  - platform: template
+    id: btn_turn_off
+    name: "Turn Off"
+    on_press:
+      - light.turn_off:
+          id: test_light
+
+  # Test 3: light.turn_on with color_temperature
+  - platform: template
+    id: btn_turn_on_ct
+    name: "Turn On CT"
+    on_press:
+      - light.turn_on:
+          id: test_light
+          color_temperature: 4000 K
+          brightness: 0.8
+
+  # Test 4: light.turn_on with effect
+  - platform: template
+    id: btn_turn_on_effect
+    name: "Turn On Effect"
+    on_press:
+      - light.turn_on:
+          id: test_light
+          effect: "Test Effect"
+
+  # Test 5: light.turn_on with effect none to clear it
+  - platform: template
+    id: btn_clear_effect
+    name: "Clear Effect"
+    on_press:
+      - light.turn_on:
+          id: test_light
+          effect: "None"
+
+  # Test 6: light.control with cold/warm white
+  - platform: template
+    id: btn_control_cw
+    name: "Control CW"
+    on_press:
+      - light.control:
+          id: test_light
+          cold_white: 0.9
+          warm_white: 0.1
+
+  # Test 7: light.turn_on with lambda brightness (tests lambda path)
+  - platform: template
+    id: btn_lambda_brightness
+    name: "Lambda Brightness"
+    on_press:
+      - light.turn_on:
+          id: test_light
+          brightness: !lambda "return id(test_brightness);"
+          red: 1.0
+          green: 0.0
+          blue: 0.0
+
+  # Test 8: light.turn_on with transition_length
+  - platform: template
+    id: btn_turn_on_transition
+    name: "Turn On Transition"
+    on_press:
+      - light.turn_on:
+          id: test_light
+          brightness: 0.5
+          transition_length: 0s
+          red: 0.5
+          green: 0.5
+          blue: 0.0