feat: double-click a merge clip to set its join crossfade/curve override + row labels

tests/test_ui_structure.py

@@ -616,3 +616,24 @@ def test_merge_preview_blocks_short_clip(win, tmp_path, monkeypatch):
 def test_merge_preview_empty_is_noop(win):
     win._merge_list.clear()
     win._on_merge_preview()
+
+
+def test_merge_override_applies_and_labels(win, tmp_path):
+    from PyQt6.QtCore import Qt
+    a = tmp_path / "a.wav"; b = tmp_path / "b.wav"; a.write_bytes(b""); b.write_bytes(b"")
+    win._merge_list.clear(); win._merge_add_paths([str(a), str(b)])
+    it0 = win._merge_list.item(0)
+    win._apply_merge_override(it0, 0.8, "exp")
+    assert it0.data(Qt.ItemDataRole.UserRole + 2) == 0.8
+    assert it0.data(Qt.ItemDataRole.UserRole + 3) == "exp"
+    assert "0.80s exp" in it0.text()
+    assert "⤲" not in win._merge_list.item(1).text()   # last item shows no join info
+    xfs, cvs = win._merge_join_values()
+    assert xfs == [0.8] and cvs == ["exp"]
+
+
+def test_merge_double_click_last_is_noop(win, tmp_path):
+    a = tmp_path / "a.wav"; a.write_bytes(b"")
+    win._merge_list.clear(); win._merge_add_paths([str(a)])
+    win._on_merge_item_double_clicked(win._merge_list.item(0))   # only/last -> no dialog, no raise
+    assert win._merge_list.count() == 1