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

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

main.py

@@ -4538,6 +4538,43 @@ class MainWindow(QMainWindow):
             "Audition the current audio area (with the edits applied)")
         self._btn_audio_play.toggled.connect(self._on_audio_audition)
         self._audition_proc = None   # QProcess while auditioning, else None
+
+        # ── Merge (crossfade) pane widgets ───────────────────────────
+        self._merge_list = QListWidget()
+        self._merge_list.setSelectionMode(
+            QAbstractItemView.SelectionMode.SingleSelection)
+        self._merge_list.setToolTip("Clips to merge, top to bottom")
+        self._spn_crossfade = QDoubleSpinBox()
+        self._spn_crossfade.setRange(0.0, 10.0)
+        self._spn_crossfade.setDecimals(2)
+        self._spn_crossfade.setSingleStep(0.1)
+        self._spn_crossfade.setSuffix(" s")
+        self._spn_crossfade.setToolTip("Crossfade applied at every join (0 = butt-join)")
+        self._spn_crossfade.setValue(float(self._settings.value("audio_crossfade", 0.5)))
+        self._spn_crossfade.valueChanged.connect(
+            lambda v: self._settings.setValue("audio_crossfade", v))
+        self._btn_merge_add_sel = QPushButton("＋ Selection")
+        self._btn_merge_add_sel.setToolTip(
+            "Add the current audio area (with edits) as a clip")
+        self._btn_merge_add_sel.clicked.connect(self._on_merge_add_selection)
+        self._btn_merge_add_file = QPushButton("＋ File…")
+        self._btn_merge_add_file.setToolTip("Add audio file(s) as clips")
+        self._btn_merge_add_file.clicked.connect(self._on_merge_add_file)
+        self._btn_merge_up = QPushButton("▲")
+        self._btn_merge_up.setToolTip("Move selected clip up")
+        self._btn_merge_up.clicked.connect(lambda: self._merge_move(-1))
+        self._btn_merge_down = QPushButton("▼")
+        self._btn_merge_down.setToolTip("Move selected clip down")
+        self._btn_merge_down.clicked.connect(lambda: self._merge_move(1))
+        self._btn_merge_remove = QPushButton("✕")
+        self._btn_merge_remove.setToolTip("Remove selected clip")
+        self._btn_merge_remove.clicked.connect(self._merge_remove_selected)
+        self._btn_merge_preview = QPushButton("▶ Preview")
+        self._btn_merge_preview.setToolTip("Merge to a temp file and audition")
+        self._btn_merge_preview.clicked.connect(self._on_merge_preview)
+        self._btn_merge_save = QPushButton("Merge && Save…")
+        self._btn_merge_save.setToolTip("Merge the clips and save the result")
+        self._btn_merge_save.clicked.connect(self._on_merge_save)
         self._transport_row = transport_row
 
         # Row 1b — subcategory (subprofile) export buttons live on their own
@@ -4864,6 +4901,27 @@ class MainWindow(QMainWindow):
         sg.setColumnStretch(3, 1)
         tb.addItem(scan_page, "Scan / Classify")
 
+        # ── Page 3: Merge (crossfade) ────────────────────────────
+        merge_page = QWidget()
+        mv = QVBoxLayout(merge_page)
+        mv.setContentsMargins(8, 6, 8, 6); mv.setSpacing(6)
+        mv.addWidget(self._merge_list)
+        xrow = QHBoxLayout()
+        xrow.addWidget(QLabel("Crossfade:"))
+        xrow.addWidget(self._spn_crossfade)
+        xrow.addStretch()
+        mv.addLayout(xrow)
+        brow = QHBoxLayout()
+        for b in (self._btn_merge_add_sel, self._btn_merge_add_file,
+                  self._btn_merge_up, self._btn_merge_down, self._btn_merge_remove):
+            brow.addWidget(b)
+        mv.addLayout(brow)
+        arow = QHBoxLayout()
+        arow.addWidget(self._btn_merge_preview)
+        arow.addWidget(self._btn_merge_save)
+        mv.addLayout(arow)
+        tb.addItem(merge_page, "Merge")
+
         outer.addWidget(tb)
 
     # ── Menu bar ─────────────────────────────────────────────
@@ -6521,6 +6579,24 @@ class MainWindow(QMainWindow):
         start = self._cursor
         self._timeline.set_audio_region(start, start + self._spn_audio_len.value())
 
+    def _on_merge_add_selection(self) -> None:
+        pass   # implemented in M3
+
+    def _on_merge_add_file(self) -> None:
+        pass   # implemented in M3
+
+    def _merge_move(self, delta: int) -> None:
+        pass   # implemented in M3
+
+    def _merge_remove_selected(self) -> None:
+        pass   # implemented in M3
+
+    def _on_merge_preview(self) -> None:
+        pass   # implemented in M4
+
+    def _on_merge_save(self) -> None:
+        pass   # implemented in M4
+
     def _on_wave_refresh(self) -> None:
         """Decode the current audio area and repaint the waveform strip."""
         if not self._file_path: