feat: global crossfade curve combo + per-join render/guard plumbing

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

tests/test_ui_structure.py

@@ -532,18 +532,48 @@ def test_merge_save_builds_command(win, tmp_path, monkeypatch):
     win._spn_crossfade.setValue(0.75)
     seen = {}
     class _Stop(Exception): pass
-    def fake(clips, xf, out):
-        seen.update(clips=clips, xf=xf, out=out); raise _Stop
+    def fake(clips, xf, out, curves=None):
+        seen.update(clips=clips, xf=xf, out=out, curves=curves); raise _Stop
     monkeypatch.setattr(m, "build_crossfade_merge_command", fake)
     monkeypatch.setattr(m.QFileDialog, "getSaveFileName",
                         staticmethod(lambda *a, **k: (str(tmp_path / "m.wav"), "")))
     with pytest.raises(_Stop):
         win._on_merge_save()
     assert seen["clips"] == [str(a), str(b)]
-    assert seen["xf"] == 0.75
+    assert seen["xf"] == [0.75]           # 1 join for 2 clips
+    assert seen["curves"] == ["tri"]
     assert seen["out"].endswith(".wav")
 
 
+def test_merge_curve_combo_present(win):
+    from PyQt6.QtWidgets import QComboBox
+    assert isinstance(win._cmb_curve, QComboBox)
+    assert win._cmb_curve.currentData() == "tri"    # default
+
+
+def test_merge_render_builds_per_join_lists(win, tmp_path, monkeypatch):
+    import main as m, pytest
+    from PyQt6.QtCore import Qt
+    a = tmp_path / "a.wav"; b = tmp_path / "b.wav"; c = tmp_path / "c.wav"
+    for f in (a, b, c): f.write_bytes(b"")
+    win._merge_list.clear()
+    win._merge_add_paths([str(a), str(b), str(c)])
+    for i in range(3):
+        win._merge_list.item(i).setData(Qt.ItemDataRole.UserRole + 1, 5.0)  # long enough
+    win._spn_crossfade.setValue(0.5)
+    seen = {}
+    class _Stop(Exception): pass
+    def fake(clips, xf, out, curves=None):
+        seen.update(clips=clips, xf=xf, out=out, curves=curves); raise _Stop
+    monkeypatch.setattr(m, "build_crossfade_merge_command", fake)
+    monkeypatch.setattr(m.QFileDialog, "getSaveFileName",
+                        staticmethod(lambda *a, **k: (str(tmp_path / "m.wav"), "")))
+    with pytest.raises(_Stop):
+        win._on_merge_save()
+    assert seen["xf"] == [0.5, 0.5]        # 2 joins for 3 clips, effective = global
+    assert seen["curves"] == ["tri", "tri"]
+
+
 def test_merge_save_blocks_short_clip(win, tmp_path, monkeypatch):
     import main as m
     from PyQt6.QtCore import Qt