feat(media_player): source list + native select_source from the app list (t47 A)

The last piece a universal-media-player replacement needs: a dropdown of the apps,
an answer to what is on screen, and a way to pick one — all reading ONE app directory
(the user's Apps map, else DEFAULT_APP_DIRECTORY), so the dropdown can never offer a
name the launch path cannot resolve. That invariant is pinned by a test that walks
source_list and resolves every entry.

* `source_list` — friendly names in directory order (the user's map order, or the
  curated list), from a new `_app_directory` accessor that the browse tree now shares.
* `source` — the foreground app's resolved name via the existing `resolve_app_name`
  chain (map → cached device-manifest label → raw package), i.e. exactly `app_name`,
  so the media player and the Current App sensor cannot disagree.
* `select_source(name)` — a configured select_source script still takes precedence
  (t44, unchanged: selecting a source can be more than launching an app); without one
  the name is resolved back to its package and launched over the shared
  `play_media("app", …)` path, so the native semantics are the ones play_media already
  had. Accepts a raw package id (what the browse tree publishes) and matches names
  case-insensitively; an unknown or empty name raises
  ServiceValidationError(translation_domain="nvidia_shield",
  translation_key="unknown_source", translation_placeholders={"source": …}) — a
  translated error, never a silent no-op — with new copy in BOTH strings files.
* SELECT_SOURCE is advertised when EITHER a select_source script is configured OR
  source_list is non-empty; VOLUME_SET stays script-only (the protocol has no
  absolute-volume key), and the two flags are pinned as independent.

Two t44 tests were updated because this is a deliberate behaviour change: "no native
path" is now only true of volume_set, and SELECT_SOURCE is advertised by default
because the curated directory always offers something.

Also: rewriting the strings files re-indented the seven script labels added in t44
(they were valid JSON but sat two levels shallow); the diff there is that fix plus
the new key.

Verified: 441 passed / 2 skipped, ruff clean, drift engine exit 0 under real HA
(23 modules, 59 imports, 157 relative imports, 36 call arguments, 7 awaited HA calls).

custom_components/nvidia_shield/strings.json

@@ -107,13 +107,13 @@
           "channel_digit_delay": "Channel digit delay (seconds)",
           "channel_send_enter": "Send ENTER after channel digits",
           "script_turn_on": "Turn on script",
-      "script_turn_off": "Turn off script",
-      "script_volume_up": "Volume up script",
-      "script_volume_down": "Volume down script",
-      "script_volume_set": "Volume set script",
-      "script_volume_mute": "Mute script",
-      "script_select_source": "Select source script",
-      "logcat_events": "Instant app detection (logcat stream)"
+          "script_turn_off": "Turn off script",
+          "script_volume_up": "Volume up script",
+          "script_volume_down": "Volume down script",
+          "script_volume_set": "Volume set script",
+          "script_volume_mute": "Mute script",
+          "script_select_source": "Select source script",
+          "logcat_events": "Instant app detection (logcat stream)"
         },
         "data_description": {
           "jellyfin_entity": "The Jellyfin media_player for this Shield. Stream details come from the HA Jellyfin integration \u2014 no extra setup needed.",
@@ -162,6 +162,10 @@
     "adb_unavailable": {
       "title": "ADB unavailable",
       "message": "The ADB connection of {entity_id} is down \u2014 command not sent."
+    },
+    "unknown_source": {
+      "title": "Unknown source",
+      "message": "'{source}' is not one of this Shield's apps. Use a name from the source list \u2014 the app names from the Apps map, or the built-in list when no map is set."
     }
   },
   "entity": {