Add drag auto-scroll near the list edges

While a track drag hovers within 28px of the track list's top/bottom
edge, the list scrolls that way (40ms timer) so the user can drop onto
off-screen rows. Qt's built-in autoscroll never fires because our
dragMoveEvent accepts the event itself, so this is driven manually; the
drop line refreshes as content scrolls under a stationary cursor.

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

tests/test_round13.py

@@ -0,0 +1,71 @@
+"""Round 13: drag auto-scroll.
+
+While dragging a track near the top/bottom edge of the list, it scrolls in
+that direction so off-screen rows can be reached. Qt's built-in autoscroll
+never runs (our dragMoveEvent accepts the event itself), so this is manual.
+"""
+
+from PyQt6.QtCore import QPoint
+
+from lintunes.gui.track_table import TrackTableView, autoscroll_direction
+
+
+class TestAutoscrollDirection:
+    def test_top_edge_scrolls_up(self):
+        assert autoscroll_direction(5, height=200, accepted=True) == -1
+
+    def test_bottom_edge_scrolls_down(self):
+        assert autoscroll_direction(195, height=200, accepted=True) == 1
+
+    def test_middle_does_not_scroll(self):
+        assert autoscroll_direction(100, height=200, accepted=True) == 0
+
+    def test_not_a_drop_target_never_scrolls(self):
+        assert autoscroll_direction(5, height=200, accepted=False) == 0
+        assert autoscroll_direction(195, height=200, accepted=False) == 0
+
+
+class TestAutoscrollTick:
+    def test_tick_scrolls_by_direction(self, qapp):
+        view = TrackTableView(playlist_mode=True)
+        bar = view.verticalScrollBar()
+        bar.setRange(0, 50)
+        bar.setValue(10)
+
+        view._autoscroll_dir = 1
+        view._autoscroll_tick()
+        assert bar.value() == 11
+
+        view._autoscroll_dir = -1
+        view._autoscroll_tick()
+        assert bar.value() == 10
+
+    def test_tick_stops_at_the_end(self, qapp):
+        view = TrackTableView(playlist_mode=True)
+        bar = view.verticalScrollBar()
+        bar.setRange(0, 50)
+        bar.setValue(50)  # already at the bottom
+
+        view._autoscroll_dir = 1
+        view._autoscroll_timer.start()
+        view._autoscroll_tick()
+
+        assert bar.value() == 50
+        assert not view._autoscroll_timer.isActive()
+
+
+class TestUpdateAutoscroll:
+    def test_rejected_drop_keeps_timer_off(self, qapp):
+        view = TrackTableView(playlist_mode=True)
+        view._update_autoscroll(QPoint(10, 5), accepted=False)
+        assert view._autoscroll_dir == 0
+        assert not view._autoscroll_timer.isActive()
+
+    def test_stop_autoscroll_resets(self, qapp):
+        view = TrackTableView(playlist_mode=True)
+        view._autoscroll_dir = 1
+        view._autoscroll_timer.start()
+        view._stop_autoscroll()
+        assert view._autoscroll_dir == 0
+        assert view._autoscroll_pos is None
+        assert not view._autoscroll_timer.isActive()