feat: selection mode filtering with av fallback

tests/test_selection.py

@@ -48,3 +48,34 @@ def test_weights_scale_planes_independently():
 def test_record_coordinate_extracts_axes():
     record = make_record(left=1, right=2, dark=3, light=4)
     assert record_coordinate(record) == Coordinate(1, 2, 3, 4)
+
+
+from hef.selection import candidates_for_mode
+
+
+def test_candidates_filter_to_exact_mode():
+    records = [make_record(id="v", mode="video"), make_record(id="a", mode="audio")]
+    result = candidates_for_mode(records, "audio", pool_size=4)
+    assert [r.id for r in result] == ["a"]
+
+
+def test_av_does_not_fall_back_when_enough_av():
+    records = [make_record(id=f"av{i}", mode="av") for i in range(4)]
+    records.append(make_record(id="a", mode="audio"))
+    result = candidates_for_mode(records, "av", pool_size=4)
+    assert {r.id for r in result} == {"av0", "av1", "av2", "av3"}
+
+
+def test_av_falls_back_to_audio_and_video_when_thin():
+    records = [
+        make_record(id="av0", mode="av"),
+        make_record(id="a", mode="audio"),
+        make_record(id="v", mode="video"),
+    ]
+    result = candidates_for_mode(records, "av", pool_size=4)
+    assert {r.id for r in result} == {"av0", "a", "v"}
+
+
+def test_candidates_rejects_none_mode():
+    with pytest.raises(ValueError):
+        candidates_for_mode([], "none", pool_size=4)