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_match_primitives.py

@@ -0,0 +1,78 @@
+from __future__ import annotations
+
+import pytest
+
+from unshackle.core.utilities import is_close_match, is_exact_match
+
+
+@pytest.mark.parametrize(
+    "needle,haystack,expected",
+    [
+        ("en", ["en"], True),
+        ("fr", ["en", "de"], False),
+        ("es", ["es-419"], True),
+        ("es", ["es-ES"], True),
+        ("es-419", ["es"], True),
+        ("en", ["en-US"], True),
+        ("en-US", ["en-GB"], True),
+        ("EN", ["en"], True),
+        ("en", ["EN"], True),
+        ("ja", ["ko"], False),
+        ("fil", ["en", "fr", "de"], False),
+        ("en", [], False),
+        ("en", [None, "en"], True),
+        ("en", [None], False),
+        ("zh", ["zh-Hans"], True),
+        ("zh-CN", ["zh-Hans"], True),
+        ("zh-Hans", ["zh-CN"], True),
+        ("zh-TW", ["zh-Hant"], True),
+        ("zh-Hant", ["zh-TW"], True),
+        ("zh", ["zh-Hant"], False),
+        ("zh-Hans", ["zh-Hant"], False),
+        ("zh-CN", ["zh-TW"], False),
+        ("zh-HK", ["zh-Hant"], True),
+        ("zh", ["cmn"], True),
+        ("cmn", ["zh"], True),
+        ("zh", ["yue"], False),
+        ("yue", ["zh-HK"], False),
+        ("fil", ["tl"], True),
+        ("tl", ["fil"], True),
+        ("fil", ["tgl"], True),
+        ("tgl", ["fil"], True),
+        ("fil", ["fil-PH"], True),
+        ("tl", ["fil-PH"], True),
+    ],
+)
+def test_is_close_match(needle, haystack, expected):
+    assert is_close_match(needle, haystack) is expected
+
+
+@pytest.mark.parametrize(
+    "needle,haystack,expected",
+    [
+        ("es", ["es-419"], False),
+        ("es-419", ["es"], False),
+        ("es-419", ["es-419"], True),
+        ("en-US", ["en-GB"], False),
+        ("en-US", ["en-US"], True),
+        ("en", ["en"], True),
+        ("EN", ["en"], True),
+        ("fr", ["de"], False),
+        ("fil", ["en"], False),
+        ("en", [], False),
+        ("zh", ["zh-Hans"], True),
+        ("zh-CN", ["zh-Hans"], True),
+        ("zh-TW", ["zh-Hant"], True),
+        ("zh", ["cmn"], True),
+        ("zh-HK", ["zh-Hant"], False),
+        ("zh-Hans", ["zh-Hant"], False),
+        ("zh-CN", ["zh-TW"], False),
+        ("fil", ["tl"], True),
+        ("tl", ["fil"], True),
+        ("fil", ["tgl"], True),
+        ("fil", ["fil-PH"], True),
+        ("tl", ["fil-PH"], True),
+    ],
+)
+def test_is_exact_match(needle, haystack, expected):
+    assert is_exact_match(needle, haystack) is expected