Add volume slider to the transport bar

A slim horizontal master-volume slider in its own slot between the
visualizer and the timeline (TASKS.md). Perceptual->linear gain so it
ramps like iTunes; persists to preferences.json (debounced so it doesn't
thrash the theme). Clicking jumps to the spot via a new ClickJumpSlider
base extracted from SeekSlider.

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

tests/test_round10.py

@@ -0,0 +1,104 @@
+"""Round 10: master output volume slider in the transport bar.
+
+1. Player.set_volume / volume round-trip (logical 0..1, clamped).
+2. TransportBar builds a volume slider initialized from the prefs value and
+   sets the player volume to match.
+3. Moving the slider updates the player live and (after the debounce) persists
+   the new volume to preferences.json.
+"""
+
+from unittest.mock import MagicMock, patch
+
+from lintunes.models import Library, Track
+from lintunes.library_manager import LibraryManager
+from lintunes.preferences import Preferences
+from lintunes import player as player_module
+from lintunes.player import Player
+from lintunes.gui.transport import TransportBar, ClickJumpSlider
+
+
+def _mock_player():
+    """A real Player with the Qt multimedia backend stubbed out (it blocks on
+    init headless — see tests/test_player.py). The audio output becomes a mock,
+    so set_volume's setVolume call is a harmless no-op."""
+    library = Library(tracks={1: Track(track_id=1, name="One")})
+    with patch.multiple(
+        player_module,
+        QMediaPlayer=MagicMock(),
+        QAudioOutput=MagicMock(),
+        QAudioBufferOutput=MagicMock(),
+        QMediaDevices=MagicMock(),
+    ):
+        return Player(MagicMock(library=library))
+
+
+# --------------------------------------------------------------------------
+# 1. Player volume API
+# --------------------------------------------------------------------------
+
+class TestPlayerVolume:
+    def test_round_trip(self, qapp):
+        player = _mock_player()
+        player.set_volume(0.5)
+        assert player.volume() == 0.5
+        # The linear gain handed to QAudioOutput is set (perceptual->linear).
+        player._audio.setVolume.assert_called()
+
+    def test_clamps_out_of_range(self, qapp):
+        player = _mock_player()
+        player.set_volume(1.5)
+        assert player.volume() == 1.0
+        player.set_volume(-0.3)
+        assert player.volume() == 0.0
+
+
+# --------------------------------------------------------------------------
+# 2 + 3. TransportBar volume slider
+# --------------------------------------------------------------------------
+
+class TestTransportVolumeSlider:
+    def _build(self, tmp_path, prefs):
+        player = _mock_player()
+        manager = LibraryManager(
+            Library(tracks={1: Track(track_id=1, name="One")}), tmp_path)
+        return TransportBar(player, manager, prefs), player
+
+    def test_slider_initialized_from_prefs(self, qapp, tmp_path):
+        prefs = Preferences(tmp_path)
+        prefs.set("volume", 0.3)
+        transport, player = self._build(tmp_path, prefs)
+
+        assert transport._volume_slider.value() == 30
+        assert player.volume() == 0.3
+
+    def test_moving_slider_updates_player_live(self, qapp, tmp_path):
+        prefs = Preferences(tmp_path)
+        transport, player = self._build(tmp_path, prefs)
+
+        transport._volume_slider.setValue(20)
+
+        assert player.volume() == 0.2
+
+    def test_save_persists_to_disk(self, qapp, tmp_path):
+        prefs = Preferences(tmp_path)
+        transport, player = self._build(tmp_path, prefs)
+
+        transport._volume_slider.setValue(20)
+        # The debounce timer normally fires this; call it directly.
+        transport._save_volume()
+
+        # A freshly loaded Preferences sees the persisted value.
+        assert Preferences(tmp_path).get("volume") == 0.2
+
+    def test_click_jumps_to_position(self, qapp, tmp_path):
+        """Click-to-jump: the slider lands on the clicked spot, not a step."""
+        prefs = Preferences(tmp_path)
+        transport, player = self._build(tmp_path, prefs)
+        slider = transport._volume_slider
+
+        assert isinstance(slider, ClickJumpSlider)
+        slider.resize(100, 20)
+        # A click at the far-left maps to the minimum, mid to ~middle.
+        assert slider._value_at(0) == 0
+        assert 40 <= slider._value_at(50) <= 60
+        assert slider._value_at(100) == 100