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).

tests/entity_stublib.py

@@ -365,6 +365,7 @@ def install() -> None:
             super().__init__(*args or (kwargs.get("translation_key"),))
             self.translation_domain = kwargs.get("translation_domain")
             self.translation_key = kwargs.get("translation_key")
+            self.translation_placeholders = kwargs.get("translation_placeholders")
 
     class ServiceValidationErrorStubError(HomeAssistantErrorStubError):
         """Real HA: ServiceValidationError derives from HomeAssistantError (t44)."""