docs: state ladder + fallback semantics in README; mediasession2mqtt retirement + universal-player glue appendix in RESEARCH_SHIELD; TiviMate as the documented no-session case (P3-3d)

docs/RESEARCH_SHIELD.md

@@ -394,3 +394,30 @@ media_player:
 9. Migration note: retire mediasession2mqtt sensors + universal-player YAML once
    Phase 3 lands (documented in README).
 10. Netflix stale-session fixture pins "inactive no-state sessions ignored".
+
+---
+
+## Appendix — mediasession2mqtt retirement + universal-player glue (Phase 3)
+
+The fork retires the surrounding glue once the state ladder (README, Phase 3)
+covers what the external pieces provided:
+
+| External piece | What it provided | Now native (file) |
+|---|---|---|
+| `mediasession2mqtt` bridge sensors | per-app now-playing pushed over MQTT from the Shield's notification listener | ADB media-session polling + the foreground tier (`connection.py`, `media_player.py`); the `media_dag_now_playing` attribute carries `source: jellyfin \| media_session \| app` |
+| Universal media player `media_dag_running_apps` template | app switcher list with playback states | `running_apps_from_tasks(..., last_known=...)` (`ha_adb_common/running_apps.py`) — enriched with `foreground`/`state`/`last_known_state`/`last_known_title` |
+| Universal player "currently in app X" template | fallback state for no-session apps | the foreground ladder tier (`media_player.py` state property, tier 3.5) |
+| External androidtv_remote entity for commands | remote keys | the protocol-backed `remote` entity (Phase 2) — the external entity remains only as the transitional last command tier, removed in Phase 4 |
+| Last-channel / "recently played" sensors | what was on when the session vanished | the TTL-capped last-known-state cache + `_last_channel` + the audio-playback hint (`connection.py`) |
+
+The migration path: point the automations/scripts at the integration's DAG
+attributes (`media_dag_*`) instead of the MQTT sensors, then remove
+`mediasession2mqtt` and the universal-player config entries. No YAML glue is
+needed for the MVP feature set; the appendix exists so retirement can be
+staged per household.
+
+**TiviMate (`ar.tvplayer.tv`)** is the documented no-session case (R2): it
+publishes no MediaSession on Shield Android 11. Its visibility comes from the
+foreground tier (tier 3.5) with the audio-playback hint and the
+last-known-state cache — never from a session, never resurrected past the
+state TTL.