Split now playing, the quiz buttons and song loading out of the quiz

NowPlaying, QuizToolbar and SongControls become their own scenes, the auto
repeat switch moves into the music player scene, and downloading songs moves
into a SongLoader node. The quiz only wires the parts together. Every node
keeps its place on screen.

Screenshots of every UI state are pixel-identical to before, the layout test
still matches the original baseline, and each new part has its own tests.

tests/integration/test_song_loader.gd

@@ -0,0 +1,122 @@
+extends GdUnitTestSuite
+## Downloading songs into the Playlist ahead of time, against a fake server.
+
+const SONG_INFO: Dictionary = {"SongNo": 0, "Soundtrack": "Chrono Trigger", "Song": "Corridors of Time"}
+
+var _snapshot: AutoloadSnapshot
+var _server: FakeHttpTransport
+var _loader: SongLoader
+
+
+func before_test() -> void:
+	_snapshot = AutoloadSnapshot.take([Settings, Playlist])
+	Playlist.reset_playlist()
+	Playlist.is_song_downloading = false
+	Playlist.number_of_tracks_to_preload = 1
+	Settings.use_low_played_mode = false
+	_server = FakeHttpTransport.new().install()
+	_server.respond_json("/music/list", [])
+	_server.respond_bytes("/music/rand", FileAccess.get_file_as_bytes("res://tests/data/short.mp3"))
+	_server.respond_bytes("/music/addQue", PackedByteArray())
+	_server.respond_json("/music/info", SONG_INFO)
+	_loader = auto_free(SongLoader.new())
+	add_child(_loader)
+
+
+func after_test() -> void:
+	_server.uninstall()
+	_snapshot.restore()
+
+
+func test_load_song_list_adds_songs_already_played_on_the_server() -> void:
+	_server.respond_json("/music/list", [
+		{"SongNo": 0, "Soundtrack": "Chrono Trigger", "Song": "Corridors of Time"},
+	])
+	_server.respond_json("/music/info", {"SongNo": 1, "Soundtrack": "Final Fantasy VI", "Song": "Dancing Mad"})
+	var signals: GdUnitSignalAssert = assert_signal(monitor_signals(_loader))
+
+	_loader.load_song_list()
+
+	await signals.is_emitted("song_list_loaded")
+	var first: SongObject = Playlist.song_object_array[0]
+	assert_str(first.soundtrack_title).is_equal("Chrono Trigger")
+	assert_bool(first.has_played).is_true()
+	assert_bool(first.is_answered).is_true()
+
+
+func test_load_song_list_then_preloads_a_new_song() -> void:
+	_loader.load_song_list()
+
+	assert_int(_server.count("/music/rand")).is_equal(1)
+	assert_int(Playlist.song_object_array.size()).is_equal(1)
+
+
+func test_fetch_song_downloads_queues_and_adds_the_song() -> void:
+	var signals: GdUnitSignalAssert = assert_signal(monitor_signals(_loader))
+
+	_loader.fetch_song()
+
+	await signals.is_emitted("song_added")
+	assert_array(_server.requested_paths).contains_exactly(["/music/rand", "/music/addQue", "/music/info"])
+	var song: SongObject = Playlist.song_object_array[0]
+	assert_str(song.song_title).is_equal("Corridors of Time")
+	assert_object(song.song).is_not_null()
+	assert_bool(Playlist.is_song_downloading).is_false()
+
+
+func test_fetch_song_keeps_going_until_the_cache_is_full() -> void:
+	Playlist.number_of_tracks_to_preload = 3
+
+	_loader.fetch_song()
+
+	assert_int(Playlist.song_object_array.size()).is_equal(3)
+	assert_int(_server.count("/music/rand")).is_equal(3)
+
+
+func test_low_played_mode_uses_its_endpoint() -> void:
+	Settings.use_low_played_mode = true
+	_server.respond_bytes("/music/rand/low", FileAccess.get_file_as_bytes("res://tests/data/short.mp3"))
+
+	_loader.fetch_song()
+
+	assert_int(_server.count("/music/rand/low")).is_equal(1)
+	assert_int(_server.count("/music/rand")).is_equal(0)
+
+
+func test_fetch_song_if_idle_does_nothing_while_downloading() -> void:
+	Playlist.is_song_downloading = true
+
+	_loader.fetch_song_if_idle()
+
+	assert_int(_server.count("/music/rand")).is_equal(0)
+
+
+func test_fill_cache_does_nothing_when_the_cache_is_full() -> void:
+	_loader.fetch_song()
+
+	_loader.fill_cache()
+
+	assert_int(_server.count("/music/rand")).is_equal(1)
+
+
+func test_fill_cache_downloads_after_the_cache_size_grows() -> void:
+	_loader.fetch_song()
+	Playlist.number_of_tracks_to_preload = 2
+
+	_loader.fill_cache()
+
+	assert_int(_server.count("/music/rand")).is_equal(2)
+
+
+func test_failed_step_is_retried_alone() -> void:
+	_loader.retry_seconds = 0.05
+	_server.fail_after("/music/info", 0)
+
+	_loader.fetch_song()
+	assert_int(Playlist.song_object_array.size()).is_equal(0)
+	_server.respond_json("/music/info", SONG_INFO)
+	await get_tree().create_timer(0.2).timeout
+
+	assert_int(_server.count("/music/rand")).is_equal(1)
+	assert_int(_server.count("/music/addQue")).is_equal(1)
+	assert_int(Playlist.song_object_array.size()).is_equal(1)