feat: play/stop audition of the current audio region

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

tests/test_ui_structure.py

@@ -386,3 +386,19 @@ def test_waveform_strip_present_and_safe(win):
     # refresh with no file loaded is a safe no-op
     win._file_path = ""
     win._on_wave_refresh()
+
+
+def test_audition_button_present_and_safe(win):
+    from PyQt6.QtWidgets import QPushButton
+    assert isinstance(win._btn_audio_play, QPushButton)
+    assert win._btn_audio_play.isCheckable()
+    assert win._btn_audio_play.text() == "▶ Play"
+    # No file loaded -> toggling on must be a safe no-op that snaps back to unchecked.
+    win._file_path = ""
+    win._btn_audio_play.setChecked(True)     # fires _on_audio_audition(True)
+    assert win._btn_audio_play.isChecked() is False
+    # Stopping when nothing is playing must be safe.
+    win._stop_audition()
+    assert win._btn_audio_play.isChecked() is False
+    assert win._btn_audio_play.text() == "▶ Play"
+    assert win._audition_proc is None