feat(tracks): add language_priority to sort preferred languages first

subtitle.language_priority and audio.language_priority prepend to the CLI
languages, so the named languages sort first and nothing is filtered.
--exact-lang now applies to the sorts too, through matching_languages, which
follows the same RFC 4647 rule as Tracks.by_language and ignores invalid tags.

tests/lang_selection/test_matching_languages.py

@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+from typing import Sequence
+
+import pytest
+
+from unshackle.core.utilities import matching_languages
+
+POOL = ["ar", "en", "en-GB", "en-US", "ja"]
+
+
+@pytest.mark.parametrize(
+    ("language", "available", "exact", "expected"),
+    [
+        ("en", POOL, False, {"en", "en-GB", "en-US"}),
+        # CLDR rates en/en-US at distance 0, so exact mode leans on the RFC 4647 string preference
+        ("en", POOL, True, {"en"}),
+        ("en-GB", POOL, True, {"en-GB"}),
+        # no string-equal tag, so exact mode falls back to the distance match
+        ("zh", ["ar", "cmn", "ja"], True, {"cmn"}),
+        ("en", ["ar", "ja"], False, set()),
+        ("en", [], False, set()),
+        ("en", ["ar", None, "en"], True, {"en"}),
+        # unparseable tags (config typos) must match nothing, never raise
+        ("engrish", POOL, False, set()),
+        ("engrish", POOL, True, set()),
+        ("", POOL, False, set()),
+        ("", POOL, True, set()),
+        ("f", POOL, False, set()),
+    ],
+)
+def test_matching_languages(language: str, available: Sequence[str], exact: bool, expected: set[str]) -> None:
+    assert matching_languages(language, available, exact) == expected

tests/lang_selection/test_resolve_sort_langs.py

@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+from typing import Optional, Sequence
+
+import pytest
+from langcodes import Language
+
+from unshackle.core.utilities import resolve_sort_langs
+
+
+@pytest.mark.parametrize(
+    ("tokens", "original", "expected"),
+    [
+        (["orig", "en", "es"], "ja", ["ja", "en", "es"]),
+        (["orig", "en", "es"], None, ["en", "es"]),
+        (["orig"], Language.get("pt-BR"), ["pt-BR"]),
+        (["en", "orig"], "en", ["en"]),
+        (["en", "es", "en", "fr"], None, ["en", "es", "fr"]),
+        (["all", "best", "en"], "ja", ["all", "best", "en"]),
+        ([], "ja", []),
+    ],
+)
+def test_resolve_sort_langs(tokens: Sequence[str], original: Optional[str], expected: list[str]) -> None:
+    assert resolve_sort_langs(tokens, original) == expected