refactor(tracks): honour the best token and share one match rule

sort_subtitles resolved only the all token, while the video and audio sorts
resolved all and best. by_language now asks matching_languages rather than
repeating the RFC 4647 preference, with no behaviour change.

tests/tracks/test_subtitle_sort.py

@@ -180,11 +180,18 @@ def test_no_priority_leaves_order_unchanged(many: Tracks) -> None:
     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")
+@pytest.mark.parametrize("token", ["all", "best"])
+def test_magic_token_alone_floats_the_original_language(many: Tracks, token: str) -> None:
+    many.sort_subtitles(by_language=[token], group_by="language")
     assert langs(many) == ["ja", "ar", "bg", "en", "es", "fr"]
 
 
+@pytest.mark.parametrize("token", ["all", "best"])
+def test_priority_outranks_either_magic_token(many: Tracks, token: str) -> None:
+    many.sort_subtitles(by_language=["en", "es", token], group_by="language")
+    assert langs(many) == ["en", "es", "ja", "ar", "bg", "fr"]
+
+
 @pytest.fixture
 def regionals() -> Tracks:
     t = Tracks()

unshackle/core/tracks/tracks.py

@@ -7,7 +7,7 @@ from functools import partial
 from pathlib import Path
 from typing import Any, Callable, Iterator, Optional, Sequence, Union
 
-from langcodes import Language, closest_supported_match
+from langcodes import Language
 from rich.progress import Progress, SpinnerColumn, TaskID, TextColumn, TimeRemainingColumn
 from rich.table import Table
 from rich.tree import Tree
@@ -15,7 +15,7 @@ from rich.tree import Tree
 from unshackle.core import binaries
 from unshackle.core.config import config
 from unshackle.core.console import GradientPulseBarColumn, console
-from unshackle.core.constants import LANGUAGE_EXACT_DISTANCE, LANGUAGE_MAX_DISTANCE, AnyTrack, TrackT
+from unshackle.core.constants import AnyTrack, TrackT
 from unshackle.core.events import events
 from unshackle.core.tracks.attachment import Attachment
 from unshackle.core.tracks.audio import Audio
@@ -351,7 +351,7 @@ class Tracks:
             by_type()
         # sections
         for language in reversed(by_language or []):
-            if str(language) == "all":
+            if str(language) in ("all", "best"):
                 language = next((x.language for x in self.subtitles if x.is_original_lang), "")
             if not language:
                 continue
@@ -469,21 +469,11 @@ class Tracks:
     def by_language(
         tracks: list[TrackT], languages: list[str], per_language: int = 0, exact_match: bool = False
     ) -> list[TrackT]:
-        distance = LANGUAGE_EXACT_DISTANCE if exact_match else LANGUAGE_MAX_DISTANCE
         selected: list[TrackT] = []
         seen_ids: set[str] = set()
         for language in languages:
-            matches = [x for x in tracks if closest_supported_match(str(x.language), [language], distance)]
-            if exact_match and len(matches) > 1:
-                # CLDR tag_distance measures intelligibility, not tag identity: it rates a base
-                # language and its "paradigm" regional variant as the same language (distance 0
-                # for ar/ar-EG, en/en-US, pt/pt-BR), so exact mode alone cannot separate them.
-                # Follow RFC 4647 Lookup: prefer the most specific (string-equal) tag, and fall
-                # back to the fuzzy match only when none exists (e.g. zh matches cmn).
-                want = Language.get(language).to_tag().casefold()
-                exact_hits = [x for x in matches if Language.get(str(x.language)).to_tag().casefold() == want]
-                if exact_hits:
-                    matches = exact_hits
+            wanted = matching_languages(language, [x.language for x in tracks], exact_match)
+            matches = [x for x in tracks if str(x.language) in wanted]
             # Overlapping tags can still resolve to the same physical track; dedupe by id so
             # callers never get duplicate tracks.
             for track in matches[: per_language or None]: