Add a SCENE platform (scene.py) so each scene preset becomes a scene.*
entity. Alexa / Google Assistant / HomeKit expose the scene, light, and
switch domains — but NOT button — so the existing button.* scene entities
are invisible to voice assistants. Scenes use a distinct unique_id so both
coexist (buttons for dashboards, scenes for voice).
- Register Platform.SCENE; bump manifest 0.4.0 -> 0.5.0
- README: "Voice Control (Alexa / Google / Siri)" guide (Nabu Casa cloud,
manual skill linking, HomeKit bridge, voice-friendly naming)
