Add Ctrl-X cut and paste-above-selection

Cut (Ctrl-X) marks the selection on the clipboard with a `cut` flag; a
paste of a cut from a real playlist is a *move* (single-undo across
playlists via move_tracks_between_playlists; same-playlist reorders),
while a copy — or a cut from the library, which has nothing to remove —
just adds. Paste now inserts *above the selected track* rather than
appending. Cut/Paste added to the track context menu; parse_tracks_mime
now tolerates a None/empty clipboard.

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

tests/test_round12.py

@@ -0,0 +1,147 @@
+"""Round 12: cut (Ctrl-X) + paste-above-selection.
+
+- The clipboard MIME carries a `cut` flag; a cut from a real playlist is a
+  *move* on paste, a copy (or a cut from the library) just adds.
+- Paste inserts *above the currently selected track*, not at the end.
+- A cross-playlist move is one undo step (move_tracks_between_playlists).
+"""
+
+from PyQt6.QtWidgets import QApplication
+
+from lintunes.models import Library, Track
+from lintunes.library_manager import LibraryManager
+from lintunes.gui.track_table import (
+    TrackTableView, make_tracks_mime, parse_tracks_mime)
+from lintunes.gui.playlist_view import PlaylistView
+
+
+def _manager(tmp_path, n=4):
+    library = Library(tracks={i: Track(track_id=i, name=f"T{i}")
+                              for i in range(1, n + 1)})
+    return LibraryManager(library, tmp_path)
+
+
+def _select_source_row(table, source_row):
+    """Select the visual row backing a given source-model row (sort-proof)."""
+    proxy_index = table.proxy.mapFromSource(table.model_.index(source_row, 0))
+    table.selectRow(proxy_index.row())
+
+
+# --------------------------------------------------------------------------
+# MIME + table helpers
+# --------------------------------------------------------------------------
+
+class TestMimeAndHelpers:
+    def test_cut_flag_round_trips(self):
+        payload = parse_tracks_mime(
+            make_tracks_mime([1, 2], "PID", [0, 1], cut=True))
+        assert payload["cut"] is True
+        assert payload["track_ids"] == [1, 2]
+        assert payload["source_playlist"] == "PID"
+        assert payload["rows"] == [0, 1]
+
+    def test_copy_defaults_to_not_cut(self):
+        payload = parse_tracks_mime(make_tracks_mime([1]))
+        assert payload["cut"] is False
+
+    def test_paste_anchor_row(self, qapp, tmp_path):
+        manager = _manager(tmp_path)
+        table = TrackTableView(playlist_mode=True)
+        table.set_tracks([manager.library.tracks[i] for i in (1, 2, 3)])
+        assert table.paste_anchor_row() is None        # nothing selected
+        _select_source_row(table, 1)
+        assert table.paste_anchor_row() == 1
+
+    def test_cut_selection_marks_clipboard(self, qapp, tmp_path):
+        manager = _manager(tmp_path)
+        table = TrackTableView(playlist_mode=True)
+        table.set_source_playlist("PID")
+        table.set_tracks([manager.library.tracks[i] for i in (1, 2, 3)])
+        _select_source_row(table, 0)
+        table.cut_selection()
+        payload = parse_tracks_mime(QApplication.clipboard().mimeData())
+        assert payload["cut"] is True
+        assert payload["track_ids"] == [1]
+
+
+# --------------------------------------------------------------------------
+# manager: single-undo cross-playlist move
+# --------------------------------------------------------------------------
+
+class TestMoveBetweenPlaylists:
+    def test_move_and_single_undo(self, tmp_path):
+        manager = _manager(tmp_path)
+        a = manager.create_playlist("A")
+        b = manager.create_playlist("B")
+        manager.add_tracks_to_playlist(a.persistent_id, [1, 2, 3])
+        manager.add_tracks_to_playlist(b.persistent_id, [4])
+
+        manager.move_tracks_between_playlists(
+            a.persistent_id, [0, 1], b.persistent_id, 0)
+
+        assert a.track_ids == [3]
+        assert b.track_ids == [1, 2, 4]
+
+        manager.undo_stack.undo()  # one step reverts both playlists
+        assert a.track_ids == [1, 2, 3]
+        assert b.track_ids == [4]
+
+    def test_same_playlist_is_a_noop(self, tmp_path):
+        manager = _manager(tmp_path)
+        a = manager.create_playlist("A")
+        manager.add_tracks_to_playlist(a.persistent_id, [1, 2, 3])
+        manager.move_tracks_between_playlists(
+            a.persistent_id, [0], a.persistent_id, 2)
+        assert a.track_ids == [1, 2, 3]
+
+
+# --------------------------------------------------------------------------
+# PlaylistView paste integration
+# --------------------------------------------------------------------------
+
+class TestPasteIntegration:
+    def test_copy_paste_inserts_above_selection(self, qapp, tmp_path):
+        manager = _manager(tmp_path)
+        b = manager.create_playlist("B")
+        manager.add_tracks_to_playlist(b.persistent_id, [4])
+        view = PlaylistView(manager)
+        view.show_playlist(b)
+
+        QApplication.clipboard().setMimeData(make_tracks_mime([1, 2]))
+        _select_source_row(view.table, 0)  # above track 4
+        view._on_paste()
+
+        assert b.track_ids == [1, 2, 4]
+
+    def test_cut_paste_moves_across_playlists(self, qapp, tmp_path):
+        manager = _manager(tmp_path)
+        a = manager.create_playlist("A")
+        b = manager.create_playlist("B")
+        manager.add_tracks_to_playlist(a.persistent_id, [1, 2, 3])
+        manager.add_tracks_to_playlist(b.persistent_id, [4])
+        view = PlaylistView(manager)
+        view.show_playlist(b)
+
+        QApplication.clipboard().setMimeData(
+            make_tracks_mime([1, 2], a.persistent_id, [0, 1], cut=True))
+        _select_source_row(view.table, 0)  # above track 4
+        view._on_paste()
+
+        assert b.track_ids == [1, 2, 4]
+        assert a.track_ids == [3]
+        # The move consumes the clipboard so it can't be pasted again.
+        assert parse_tracks_mime(QApplication.clipboard().mimeData()) is None
+
+    def test_cut_paste_same_playlist_reorders(self, qapp, tmp_path):
+        manager = _manager(tmp_path)
+        a = manager.create_playlist("A")
+        manager.add_tracks_to_playlist(a.persistent_id, [1, 2, 3, 4])
+        view = PlaylistView(manager)
+        view.show_playlist(a)
+
+        QApplication.clipboard().setMimeData(
+            make_tracks_mime([1], a.persistent_id, [0], cut=True))
+        _select_source_row(view.table, 2)  # above track 3
+        view._on_paste()
+
+        assert a.track_ids == [2, 1, 3, 4]