fix(lang): surface orig in errors and flag the original audio track over the API

Error messages named the resolved language for a user who only ever defaulted to orig. Audio tracks now carry is_original, resolved through Tracks.by_language so it agrees with the downloader.

tests/lang_selection/test_as_requested.py

@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+from typing import Optional
+
+import pytest
+
+from unshackle.core.utilities import as_requested
+
+
+@pytest.mark.parametrize(
+    "tokens,orig_token,expected",
+    [
+        # the user passed no -l, so orig resolved to en behind their back; report orig
+        (["en"], "en", "orig"),
+        (["en", "ja"], "en", "orig, ja"),
+        # the user named en, so report en
+        (["en"], None, "en"),
+        (["ja"], "en", "ja"),
+        (["en", "ja"], None, "en, ja"),
+        ([], "en", ""),
+    ],
+)
+def test_as_requested(tokens: list[str], orig_token: Optional[str], expected: str) -> None:
+    assert as_requested(tokens, orig_token) == expected