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`.

README.md

@@ -634,11 +634,67 @@ exactly as before.
 
 | Entity | Type | Description |
 |--------|------|-------------|
-| Media Player | `media_player` | Main entity with playback controls and metadata |
+| Media Player | `media_player` | Main entity with playback controls, metadata and the flat `now_playing_*` attributes (below) |
 | Current app (`sensor.current_app`) | `sensor` | Currently running app |
-| Now playing (`sensor.now_playing`) | `sensor` | Formatted now-playing string |
+| Now playing (`sensor.now_playing`) | `sensor` | Formatted now-playing string, with its own flat attributes (below) |
 | Pair ADB | `button` | Initiate or re-pair ADB connection |
 
+### Now-playing attributes
+
+**Since t65 the media player's now-playing reading is a set of separate
+TOP-LEVEL attributes**, not one nested object — that object read badly in YAML:
+
+| Attribute | What it holds |
+| --- | --- |
+| `now_playing_via` | which TIER produced the reading: `jellyfin`, `media_session` or `app` |
+| `now_playing_state` | the reading's own state: `playing` / `paused` / `buffering` / `idle` |
+| `now_playing_title` | the title — or, in privacy mode, the app identity |
+| `now_playing_artist`, `now_playing_album` | the session's artist and album, when it has them |
+| `now_playing_media_type` | the content type the source reported |
+| `now_playing_series`, `now_playing_season`, `now_playing_episode` | series metadata, when the source reports it |
+| `now_playing_duration`, `now_playing_position`, `now_playing_position_updated_at` | playback timing |
+| `now_playing_thumbnail` | the artwork URL (or the Jellyfin entity picture) |
+| `now_playing_media_info` | the Jellyfin media-info dict, when that entity has one |
+
+The rules that keep the set predictable:
+
+- **Only what the tier answered is published.** An unknown field is OMITTED, never
+  `null`, so nothing ever reads as `now_playing_title: null`.
+- **A device that reads `off` publishes none of them.** A Jellyfin session that has
+  gone stale must not put a title beside a device that is off, and the state
+  already says nothing is playing. `on` (no source reported a power or playback
+  fact) publishes none either — there is no reading to publish.
+- **`now_playing_via` is deliberately not `source`.** On a media player, `source`
+  and `source_list` mean the selected app input (`Netflix`, `TiviMate`, …) and they
+  still do; naming the tier `source` is exactly what made the reading look like
+  media metadata from somewhere else. `media_dag_role` keeps its own separate
+  meaning (that this entity is the "source" role of the media graph).
+- **`now_playing_state` IS the entity's own state**, so a dashboard never reads two
+  different answers: the state ladder is the authority, and the foreground tier's
+  last-known-playback bridge can still publish a title while the state is already
+  `idle`.
+
+**Compatibility: `media_dag_now_playing` is GONE** — not deprecated, removed. It
+was one object; its contents are the attributes above. No alias is kept: a nested
+copy would double every poll's payload, and these attributes are declared
+*unrecorded* for exactly that reason (they must not bloat the recorder database). A
+template that read the object needs one rename:
+
+```jinja
+{# before #}
+{{ state_attr('media_player.shield', 'media_dag_now_playing').title }}
+{# after #}
+{{ state_attr('media_player.shield', 'now_playing_title') }}
+```
+
+The Jellyfin-backed `sensor.<entry>_now_playing` keeps its own flat set
+(`playback_state`, `title`, `media_type`, `series`, `season`, `episode`,
+`duration`, `position`, `thumbnail`), and it follows the same rule: a field the
+Jellyfin entity did not answer is omitted, so there are no `title: null` rows
+there either. Its `playback_state` reads `idle` whenever that session is not
+playing. `sensor.<entry>_current_app` publishes `app_id`/`app_name`, the same
+identity the media player carries beside its now-playing attributes.
+
 ## Development
 
 - **Repository:** [Gitea — git.notexpectedyet.com/notexpectedyet/ha-nvidia-shield](https://git.notexpectedyet.com/notexpectedyet/ha-nvidia-shield), branch `master`