v0.1.2: transport buttons fill their bubble as equal tap targets

Prev/play/next (and shuffle) used to hug their ~20px glyphs in the middle
of the 62px rounded box, so most of the bubble was dead space. _box() now
takes split=True: the buttons tile the bubble interior in equal shares,
expanding to the (tightened) margins with the glyph centered in each.
Minimum button widths keep the bubbles at their old footprint — no bigger,
no smaller. Round 21 tests cover the equal split, vertical fill, edge
spans, and the preserved footprint.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_round21.py

@@ -0,0 +1,75 @@
+"""Round 21: transport buttons fill their bubble as equal tap targets.
+
+The prev/play/next FlashButtons used to hug their ~20px icons in the middle
+of the 62px-tall rounded box, leaving most of the bubble unclickable. Now
+_box(split=True) tiles the bubble interior with the buttons: each gets an
+equal third, expands vertically to the margins, and keeps its glyph centered.
+The bubble itself keeps its pre-split footprint (no bigger, no smaller).
+"""
+
+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, CONTROL_HEIGHT
+
+
+def _mock_player():
+    """A real Player with the Qt multimedia backend stubbed out (it blocks on
+    init headless — see tests/test_player.py)."""
+    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))
+
+
+def _build_bar(qapp, tmp_path):
+    prefs = Preferences(tmp_path)
+    manager = LibraryManager(
+        Library(tracks={1: Track(track_id=1, name="One")}), tmp_path)
+    bar = TransportBar(_mock_player(), manager, prefs)
+    bar.resize(1200, 90)
+    bar.show()
+    qapp.processEvents()
+    return bar
+
+
+class TestTransportTapTargets:
+    def test_buttons_split_the_bubble_equally(self, qapp, tmp_path):
+        bar = _build_bar(qapp, tmp_path)
+        widths = [b.width() for b in
+                  (bar._prev_btn, bar._play_btn, bar._next_btn)]
+        assert max(widths) - min(widths) <= 1
+
+    def test_buttons_fill_the_bubble_vertically(self, qapp, tmp_path):
+        bar = _build_bar(qapp, tmp_path)
+        for btn in (bar._prev_btn, bar._play_btn, bar._next_btn,
+                    bar._shuffle_btn):
+            # Bubble height minus margins+border (4px each side).
+            assert btn.height() >= CONTROL_HEIGHT - 10
+
+    def test_buttons_span_the_bubble_horizontally(self, qapp, tmp_path):
+        bar = _build_bar(qapp, tmp_path)
+        box = bar._prev_btn.parentWidget()
+        assert bar._prev_btn.x() <= 5  # flush against the left margin
+        right_edge = bar._next_btn.x() + bar._next_btn.width()
+        assert right_edge >= box.width() - 5  # ...and the right one
+        # No stretch gaps between the buttons.
+        assert bar._play_btn.x() == bar._prev_btn.x() + bar._prev_btn.width()
+        assert bar._next_btn.x() == bar._play_btn.x() + bar._play_btn.width()
+
+    def test_bubble_keeps_its_footprint(self, qapp, tmp_path):
+        """The bubble must not grow (trav's constraint) nor shrink (the
+        minimum button widths preserve the old stretch-padded size)."""
+        bar = _build_bar(qapp, tmp_path)
+        box = bar._prev_btn.parentWidget()
+        assert box.height() == CONTROL_HEIGHT
+        assert 106 <= box.width() <= 126  # was 116 pre-split