feat(dl): gate s_lang/a_lang miss behind --best-available

Missing requested subtitle and audio languages now warn and continue when --best-available is set instead of hard-exiting. Without the flag, missing languages still produce an error and exit, matching the prior strict behavior. Audio missing-lang detection is now symmetric with subtitles.

- add find_missing_langs helper in core/utilities for reuse between s_lang and a_lang paths (skips all/best/orig sentinels)
- refactor dl.py s_lang/a_lang checks to share the helper
- add tests/lang_selection covering match primitives, helper output, and tricky langcodes corners (zh-Hans/zh-Hant/zh-CN/zh-TW/zh-HK, cmn/yue, fil/tl/tgl)
- clean up unused-var ruff F841 in tests/remote/unit/

tests/lang_selection/test_find_missing_langs.py

@@ -0,0 +1,103 @@
+from __future__ import annotations
+
+import pytest
+
+from unshackle.core.utilities import find_missing_langs
+
+
+@pytest.mark.parametrize(
+    "requested,available,expected",
+    [
+        (["en"], ["en"], []),
+        (["en", "ja"], ["en", "ja", "fr"], []),
+        (["en", "fil"], ["en"], ["fil"]),
+        (["en", "ko", "ja"], ["en"], ["ko", "ja"]),
+        (["fil"], ["en", "ja"], ["fil"]),
+        (["fil", "ko"], ["en"], ["fil", "ko"]),
+        (["es"], ["es-419"], []),
+        (["es-419"], ["es"], []),
+        (["en"], ["en-US"], []),
+        (["all"], [], []),
+        (["best"], [], []),
+        (["orig"], [], []),
+        (["all", "en"], ["en"], []),
+        (["best", "fil"], ["en"], ["fil"]),
+        ([], ["en"], []),
+        (["en"], [], ["en"]),
+        (["en"], [None, "en"], []),
+        (["en"], [None], ["en"]),
+        (["zh"], ["zh-Hans"], []),
+        (["zh-CN"], ["zh-Hans"], []),
+        (["zh-Hans"], ["zh-CN"], []),
+        (["zh-TW"], ["zh-Hant"], []),
+        (["zh-Hant"], ["zh-TW"], []),
+        (["zh"], ["zh-Hant"], ["zh"]),
+        (["zh-Hans"], ["zh-Hant"], ["zh-Hans"]),
+        (["zh-CN"], ["zh-TW"], ["zh-CN"]),
+        (["zh-HK"], ["zh-Hant"], []),
+        (["zh"], ["cmn"], []),
+        (["cmn"], ["zh"], []),
+        (["zh"], ["yue"], ["zh"]),
+        (["yue"], ["zh-HK"], ["yue"]),
+        (["fil"], ["tl"], []),
+        (["tl"], ["fil"], []),
+        (["fil"], ["tgl"], []),
+        (["tgl"], ["fil"], []),
+        (["fil"], ["fil-PH"], []),
+        (["tl"], ["fil-PH"], []),
+    ],
+)
+def test_close_match(requested, available, expected):
+    assert find_missing_langs(requested, available, exact=False) == expected
+
+
+@pytest.mark.parametrize(
+    "requested,available,expected",
+    [
+        (["es"], ["es-419"], ["es"]),
+        (["es-419"], ["es"], ["es-419"]),
+        (["en"], ["en-US"], []),
+        (["en-US"], ["en"], []),
+        (["en"], ["en-GB"], ["en"]),
+        (["en-US"], ["en-GB"], ["en-US"]),
+        (["en-US"], ["en-US"], []),
+        (["en-US", "en-GB"], ["en-US"], ["en-GB"]),
+        (["en"], ["en"], []),
+        (["all", "es-419"], ["es"], ["es-419"]),
+        (["zh"], ["zh-Hans"], []),
+        (["zh-CN"], ["zh-Hans"], []),
+        (["zh-TW"], ["zh-Hant"], []),
+        (["zh"], ["cmn"], []),
+        (["zh-HK"], ["zh-Hant"], ["zh-HK"]),
+        (["zh-Hans"], ["zh-Hant"], ["zh-Hans"]),
+        (["zh-CN"], ["zh-TW"], ["zh-CN"]),
+        (["fil"], ["tl"], []),
+        (["tl"], ["fil"], []),
+        (["fil"], ["tgl"], []),
+        (["fil"], ["fil-PH"], []),
+        (["tl"], ["fil-PH"], []),
+    ],
+)
+def test_exact_match(requested, available, expected):
+    assert find_missing_langs(requested, available, exact=True) == expected
+
+
+def test_order_preserved():
+    assert find_missing_langs(["ja", "ko", "fr"], ["en"], exact=False) == ["ja", "ko", "fr"]
+
+
+def test_duplicates_in_request():
+    assert find_missing_langs(["fil", "fil", "en"], ["en"], exact=False) == ["fil", "fil"]
+
+
+def test_zh_catalogue_simplified_only_misses_traditional():
+    assert find_missing_langs(["zh-Hant", "zh-TW"], ["en", "zh-Hans"], exact=False) == ["zh-Hant", "zh-TW"]
+
+
+def test_mixed_zh_fil_request():
+    assert find_missing_langs(["en", "zh-Hans", "fil"], ["en", "tl"], exact=False) == ["zh-Hans"]
+
+
+def test_zh_cn_request_with_tw_catalogue():
+    assert find_missing_langs(["zh-CN"], ["zh-TW"], exact=False) == ["zh-CN"]
+    assert find_missing_langs(["zh-CN"], ["zh-TW"], exact=True) == ["zh-CN"]