feat(media-player): flatten now-playing into separate attributes and name the tier

The user reads attributes as YAML — "These could do with being separate
attributes rather than an object in one?" — so the nested
`media_dag_now_playing` object is gone and every field it carried is its own
top-level attribute:

  now_playing_via, now_playing_state, now_playing_title, now_playing_artist,
  now_playing_album, now_playing_media_type, now_playing_series,
  now_playing_season, now_playing_episode, now_playing_duration,
  now_playing_position, now_playing_position_updated_at, now_playing_thumbnail,
  now_playing_media_info

Rules the surface now holds to:

- an unknown field is OMITTED, never published as None (`title: null` inside the
  object was exactly the noise this removes); 0 and False are real values
- a device reading `off` publishes no now-playing row at all — a Jellyfin session
  that has gone stale must not put a title beside a device that is off — and `on`
  (nothing reported) publishes none either, because there is no reading
- `now_playing_state` IS the entity's own state, so the ladder stays the single
  answer even while the foreground tier's last-known bridge serves a title
- the tier indicator is `now_playing_via` (`jellyfin` / `media_session` / `app`),
  NOT `source`: on a media_player `source`/`source_list` mean the selected app
  input (`Netflix`, `TiviMate`, …) and they still do — a tier named `source` is
  what made the reading look like media metadata from somewhere else. The internal
  reading key is renamed `via` as well, and the Jellyfin tier now names itself too
- every published name is declared unrecorded (G12): these change every poll

COMPATIBILITY: `media_dag_now_playing` is removed, not deprecated, and no alias is
kept — a nested copy would double every poll's payload. README documents the exact
names, these rules, the sensor's own flat set and a one-line template migration.

sensor.py: the Jellyfin now-playing sensor follows the same no-None rule (`title`
and `media_type` used to be published as null there).

Tests: 17 new cases pin the flattening, the tier rename, the five states, the
off/on gate, the playing->idle transition, the last-known bridge, privacy mode and
agreement with the Current App sensor; the tier-dict tests follow `via`.

custom_components/nvidia_shield/sensor.py

@@ -299,23 +299,24 @@ class NvidiaShieldNowPlayingSensor(NvidiaShieldSensorBase):
             return {"playback_state": "idle"}
 
         attrs = state.attributes
-        result = {
-            "playback_state": state.state,
-            "title": attrs.get("media_title"),
-            "media_type": attrs.get("media_content_type"),
-        }
-
-        if attrs.get("media_series_title"):
-            result["series"] = attrs["media_series_title"]
-        if attrs.get("media_season") is not None:
-            result["season"] = attrs["media_season"]
-        if attrs.get("media_episode") is not None:
-            result["episode"] = attrs["media_episode"]
-        if attrs.get("media_duration") is not None:
-            result["duration"] = attrs["media_duration"]
-        if attrs.get("media_position") is not None:
-            result["position"] = attrs["media_position"]
-        if attrs.get("entity_picture"):
-            result["thumbnail"] = attrs["entity_picture"]
+        # t65: the same "no None rows" rule the media player's flattened surface
+        # follows — an attribute that reads `title: null` in the template editor
+        # is noise, so a field the Jellyfin entity did not answer is OMITTED
+        # rather than published. Zero stays: 0 s of position is a real reading.
+        result = {"playback_state": state.state}
+        for key, source in (
+            ("title", "media_title"),
+            ("media_type", "media_content_type"),
+            ("series", "media_series_title"),
+            ("season", "media_season"),
+            ("episode", "media_episode"),
+            ("duration", "media_duration"),
+            ("position", "media_position"),
+            ("thumbnail", "entity_picture"),
+        ):
+            value = attrs.get(source)
+            if value is None or value == "":
+                continue
+            result[key] = value
 
         return result