feat(tracks): add subtitle.type_priority config for variant ordering

Optional ranking of forced/normal/sdh per language, mirroring audio.codec_priority; first track gets the default flag, so this also picks the default subtitle. Unset = existing behavior.

tests/tracks/test_subtitle_sort.py

@@ -0,0 +1,68 @@
+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(
+        url=f"https://example.test/{lang}-{forced}-{sdh}-{cc}",
+        language=lang,
+        codec=Subtitle.Codec.SubRip,
+        forced=forced,
+        sdh=sdh,
+        cc=cc,
+    )
+
+
+def shape(tracks: Tracks) -> list[tuple[str, str]]:
+    return [
+        (str(s.language), "forced" if s.forced else "sdh" if (s.sdh or s.cc) else "normal") for s in tracks.subtitles
+    ]
+
+
+@pytest.fixture
+def tracks() -> Tracks:
+    t = Tracks()
+    t.add([sub("en", sdh=True), sub("en", forced=True), sub("en"), sub("it", forced=True), sub("it")])
+    return t
+
+
+def test_default_order_is_forced_normal_sdh(tracks: Tracks) -> None:
+    tracks.sort_subtitles(by_language=["en"])
+    assert shape(tracks) == [
+        ("en", "forced"),
+        ("en", "normal"),
+        ("en", "sdh"),
+        ("it", "forced"),
+        ("it", "normal"),
+    ]
+
+
+def test_type_priority_overrides_group_order(tracks: Tracks) -> None:
+    tracks.sort_subtitles(by_language=["en"], type_priority=["normal", "sdh", "forced"])
+    assert shape(tracks) == [
+        ("en", "normal"),
+        ("en", "sdh"),
+        ("en", "forced"),
+        ("it", "normal"),
+        ("it", "forced"),
+    ]
+
+
+def test_type_priority_unlisted_types_fall_to_end(tracks: Tracks) -> None:
+    tracks.sort_subtitles(by_language=["en"], type_priority=["normal"])
+    # unlisted types keep their pre-sort relative order (stable sort)
+    assert shape(tracks) == [
+        ("en", "normal"),
+        ("en", "sdh"),
+        ("en", "forced"),
+        ("it", "normal"),
+        ("it", "forced"),
+    ]
+
+
+def test_cc_counts_as_sdh() -> None:
+    t = Tracks()
+    t.add([sub("en", cc=True), sub("en")])
+    t.sort_subtitles(by_language=["en"], type_priority=["normal", "sdh"])
+    assert shape(t) == [("en", "normal"), ("en", "sdh")]