feat: AudioEditorDialog scaffold + Edit-clip entry point (ops stubbed)

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

tests/test_ui_structure.py

@@ -637,3 +637,24 @@ def test_merge_double_click_last_is_noop(win, tmp_path):
     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
+
+
+def test_audio_editor_dialog_scaffold(win):
+    import main as win_module
+    from PyQt6.QtWidgets import QPushButton
+    dlg = win_module.AudioEditorDialog("/nonexistent.wav", parent=win)
+    # version stack starts with the initial file
+    assert dlg._versions == ["/nonexistent.wav"]
+    assert dlg._ver_idx == 0
+    assert dlg._current() == "/nonexistent.wav"
+    # widgets present
+    assert dlg._wave is not None
+    for name in ("_btn_delete", "_btn_silence", "_btn_reverse", "_btn_trim",
+                 "_btn_undo", "_btn_redo", "_btn_save_as"):
+        assert isinstance(getattr(dlg, name), QPushButton)
+    # undo disabled at the base version, redo disabled with no forward history
+    assert not dlg._btn_undo.isEnabled()
+    assert not dlg._btn_redo.isEnabled()
+    # a bad path decodes to an empty waveform without crashing
+    dlg._reload()
+    dlg.close()