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/tracks/test_subtitle_sort.py

@@ -3,8 +3,8 @@ import pytest
 from unshackle.core.tracks import Subtitle, Tracks
 
 
-def sub(lang: str, forced: bool = False, sdh: bool = False, cc: bool = False) -> Subtitle:
-    return Subtitle(
+def sub(lang: str, forced: bool = False, sdh: bool = False, cc: bool = False, orig: bool = False) -> Subtitle:
+    track = Subtitle(
         url=f"https://example.test/{lang}-{forced}-{sdh}-{cc}",
         language=lang,
         codec=Subtitle.Codec.SubRip,
@@ -12,6 +12,8 @@ def sub(lang: str, forced: bool = False, sdh: bool = False, cc: bool = False) ->
         sdh=sdh,
         cc=cc,
     )
+    track.is_original_lang = orig
+    return track
 
 
 def shape(tracks: Tracks) -> list[tuple[str, str]]:
@@ -135,3 +137,82 @@ def test_language_grouping_keeps_regional_variants_adjacent_through_sections() -
         ("fi", "normal"),
         ("fi", "sdh"),
     ]
+
+
+def langs(tracks: Tracks) -> list[str]:
+    return [str(s.language) for s in tracks.subtitles]
+
+
+@pytest.fixture
+def many() -> Tracks:
+    t = Tracks()
+    t.add([sub(lang, orig=lang == "ja") for lang in ("ar", "bg", "en", "es", "fr", "ja")])
+    return t
+
+
+def test_priority_orders_listed_then_rest_alphabetically(many: Tracks) -> None:
+    many.sort_subtitles(by_language=["en", "es", "fr"], group_by="language")
+    assert langs(many) == ["en", "es", "fr", "ar", "bg", "ja"]
+
+
+def test_priority_outranks_the_original_language(many: Tracks) -> None:
+    # "all" resolves to the original language (ja), which must stay below the named ones
+    many.sort_subtitles(by_language=["en", "es", "all"], group_by="language")
+    assert langs(many) == ["en", "es", "ja", "ar", "bg", "fr"]
+
+
+def test_priority_matches_regional_variants() -> None:
+    t = Tracks()
+    t.add([sub("ar"), sub("en-US"), sub("es-419"), sub("ja")])
+    t.sort_subtitles(by_language=["en", "es"], group_by="language")
+    assert langs(t) == ["en-US", "es-419", "ar", "ja"]
+
+
+def test_priority_overlapping_entries_are_harmless() -> None:
+    t = Tracks()
+    t.add([sub("ar"), sub("en-GB"), sub("en-US"), sub("es"), sub("ja")])
+    t.sort_subtitles(by_language=["en", "en-US", "es"], group_by="language")
+    assert langs(t) == ["en-GB", "en-US", "es", "ar", "ja"]
+
+
+def test_no_priority_leaves_order_unchanged(many: Tracks) -> None:
+    many.sort_subtitles(by_language=[], group_by="language")
+    assert langs(many) == ["ar", "bg", "en", "es", "fr", "ja"]
+
+
+def test_all_alone_still_floats_the_original_language(many: Tracks) -> None:
+    many.sort_subtitles(by_language=["all"], group_by="language")
+    assert langs(many) == ["ja", "ar", "bg", "en", "es", "fr"]
+
+
+@pytest.fixture
+def regionals() -> Tracks:
+    t = Tracks()
+    t.add([sub("ar"), sub("en"), sub("en-GB"), sub("en-US"), sub("ja")])
+    return t
+
+
+def test_fuzzy_priority_floats_regional_variants(regionals: Tracks) -> None:
+    regionals.sort_subtitles(by_language=["en"], group_by="language")
+    assert langs(regionals) == ["en", "en-GB", "en-US", "ar", "ja"]
+
+
+def test_exact_priority_floats_only_the_named_tag(regionals: Tracks) -> None:
+    # CLDR rates en/en-US at distance 0, so exact mode needs the RFC 4647 string preference
+    # to hold en-US back; without it en-US floats and en-GB does not.
+    regionals.sort_subtitles(by_language=["en"], group_by="language", exact_match=True)
+    assert langs(regionals) == ["en", "ar", "en-GB", "en-US", "ja"]
+
+
+def test_exact_priority_still_falls_back_when_no_tag_is_string_equal() -> None:
+    t = Tracks()
+    t.add([sub("ar"), sub("cmn"), sub("ja")])
+    t.sort_subtitles(by_language=["zh"], group_by="language", exact_match=True)
+    assert langs(t) == ["cmn", "ar", "ja"]
+
+
+@pytest.mark.parametrize("exact", [False, True])
+def test_unparseable_priority_entry_is_ignored(many: Tracks, exact: bool) -> None:
+    # a config typo in language_priority must not crash the sort
+    many.sort_subtitles(by_language=["en", "engrish", "f", ""], group_by="language", exact_match=exact)
+    assert langs(many) == ["en", "ar", "bg", "es", "fr", "ja"]