docs: correct Phase 2 volume expectations — androidtvremote2 has no absolute volume set (core closed it not_planned); VOLUME_STEP|MUTE only, level from push

docs/MVP_PLAN.md

@@ -267,10 +267,14 @@ Files:
 - `remote.py` (new platform) — protocol-backed remote entity (port of core:
   SHORT/START_LONG/END_LONG command prefixes, num_repeats/delay/hold, activity =
   current app).
-- `media_player.py` — command routing switches to protocol keys; volume gains
-  absolute level (VOLUME_SET) + real mute state; availability and is_on come
+- `media_player.py` — command routing switches to protocol keys; volume gains a
+  push-based level readout + real mute state (NOTE: androidtvremote2 has NO
+  absolute volume set — HA core closed volume_set for androidtv_remote as
+  not_planned; only VOLUME_STEP | VOLUME_MUTE are claimed, level comes from
+  volume_info push); availability and is_on come
   from protocol callbacks; `_is_power_on` ladder: protocol is_on → ADB wake →
   plug off.
+  plug off.
 - `media_player.py` `async_play_media` — §7 channel port via protocol
   `send_key_command` digits (0.1 s delay) + task-cancel guard; ADB keyevent
   fallback when the protocol is down.