fix(mux): pass the exact frame rate to mkvmerge default duration

FPS.exact keeps 24000/1001 instead of rounding to 23.976, and the hybrid source copies no longer set needs_duration_fix, which nothing read.

tests/core/test_fps.py

@@ -1,7 +1,10 @@
 import warnings
+from pathlib import Path
+from typing import Callable
 
 import pytest
 
+from unshackle.core.tracks import Tracks, Video
 from unshackle.core.utilities import FPS
 
 
@@ -27,3 +30,29 @@ def test_parse_rejects_non_numeric_constant() -> None:
 def test_parse_rejects_non_division_operation() -> None:
     with pytest.raises(ValueError, match="Invalid operation"):
         FPS.parse("24+1")
+
+
+@pytest.mark.parametrize(
+    ("fps", "expected"),
+    [
+        (24000 / 1001, "24000/1001"),
+        (23.976, "24000/1001"),
+        (23.97, "24000/1001"),
+        (59.94, "60000/1001"),
+        (24.0, "24"),
+        (25, "25"),
+        (5, "5"),
+        (12.5, "25/2"),
+    ],
+)
+def test_exact_keeps_ntsc_rates(fps: float, expected: str) -> None:
+    assert str(FPS.exact(fps)) == expected
+
+
+def test_mux_passes_the_exact_rate_as_default_duration(tmp_path: Path, mux_args: Callable[..., list[str]]) -> None:
+    video = Video(id_="v1", url="https://example.test/v", codec=Video.Codec.HEVC, language="en", fps="24000/1001")
+    video.needs_duration_fix = True
+    video.path = tmp_path / "v.hevc"
+    video.path.write_bytes(b"\x00" * 16)
+    args = mux_args(Tracks(video))
+    assert args[args.index("--default-duration") + 1] == "0:24000/1001p"