feat(const,strings,readme): adb_command consts/service name, options toggle strings, curated DEFAULT_APP_DIRECTORY and README service notes (completes P4-2 unit 1)

README.md

@@ -84,6 +84,18 @@ TV Remote entity, the Nvidia Remote Protocol (PIN pairing), or ADB
 media-session mode (ADB enabled + host). The `no_state_source` form error
 fires only when none of the three is present; ADB-only setups are valid.
 
+### Raw `adb_command` service (opt-in)
+
+When enabled in the integration's options (`adb_command_enabled`, **default
+off**), the integration registers the `nvidia_shield.adb_command` service:
+send a raw ADB shell command to the entry's persistent connection
+(`{"command": "your-command", "entity_id": "media_player.<yours>"}` — the
+entity is optional when exactly one entry provides the service). This is the
+same security posture as the Home Assistant core `androidtv` integration: a
+documented, opt-in escape hatch for debugging and automation, registered at
+setup and unregistered when the last participating entity unloads. It
+rejects (logs a warning, sends nothing) when the ADB connection is down.
+
 ### Migration from mediasession2mqtt + universal-player glue
 
 The `mediasession2mqtt` bridge sensors and the universal media player YAML