feat: Merge pane (3rd Audio QToolBox page) — list + crossfade + buttons

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_ui_structure.py

@@ -310,6 +310,16 @@ def test_audio_toolbox_has_two_panes(win):
     assert win._btn_scan in tb.findChildren(QPushButton)
 
 
+def test_merge_pane_present(win):
+    from PyQt6.QtWidgets import QToolBox, QListWidget, QDoubleSpinBox
+    tb = win._tab_audio.findChild(QToolBox)
+    titles = [tb.itemText(i) for i in range(tb.count())]
+    assert "Merge" in titles
+    assert isinstance(win._merge_list, QListWidget)
+    assert isinstance(win._spn_crossfade, QDoubleSpinBox)
+    assert win._spn_crossfade.value() == 0.5
+
+
 def test_extract_honors_format_and_edits(win, monkeypatch, tmp_path):
     import main as m
     import pytest