Add Short songs and a choice of how random songs are picked (#30, #51)

Short songs (#30): a quiz settings switch and a Seconds per song row (30 by
default, 10-120 in steps of 5), saved with the quiz settings. Only part of
each quiz song plays, from a random point 10-60% into it, picked once per song
so restart and repeat play the same part. The progress bar, time and seeking
stay within the part, a hidden beginning counts inside it, and its end counts
as the song ending (repeat, stop or go to the next song as set).

Random songs (#51): the low-played switch in Settings is now a Normal / Low
played / Classic choice, stored as Settings.random_mode and not saved.
fetch_random_song(mode) picks /music/rand, /music/rand/low or
/music/rand/classic.

Closes #30
Closes #51

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

player/music_player.gd

@@ -5,6 +5,11 @@ extends Control
 ##
 ## The buttons grow a little under the mouse and the play button pops when it switches
 ## between play and pause, see [ButtonBounce].
+##
+## With Short songs on ([member Settings.short_songs]) only a part of each quiz song plays:
+## [member Settings.short_song_seconds] from a random point between
+## [constant SHORT_START_MIN] and [constant SHORT_START_MAX] of the song. The progress bar,
+## the time and seeking all stay within that part, and its end counts as the song ending.
 
 signal update_song_list
 signal play_next_song
@@ -16,6 +21,15 @@ var playback_position: float
 var stream: AudioStream
 var song_finished: bool = false
 
+## Where in the song the part played with Short songs may start, as a share of its length.
+## Many songs have a quiet intro, so the start is never right at the beginning.
+const SHORT_START_MIN: float = 0.1
+const SHORT_START_MAX: float = 0.6
+
+# The part of the song that plays: its start in seconds, and its length (0: to the end)
+var _clip_start: float = 0.0
+var _clip_length: float = 0.0
+
 @onready var play_button: Button = %PlayButton
 @onready var restart_button: Button = %RestartButton
 @onready var audio_player: AudioStreamPlayer = $AudioStreamPlayer
@@ -38,14 +52,46 @@ func _ready() -> void:
 
 
 func _process(_delta: float) -> void:
-	if audio_player.has_stream_playback() && !is_changing && !audio_player.stream_paused:
-		if !Settings.hide_beginning \
-				|| audio_player.get_playback_position() > Settings.hide_for_seconds:
-			progress_slider.value = audio_player.get_playback_position()
-		if stream != null:
-			music_time_label.text = format_text(progress_slider.value, stream.get_length())
-		else:
-			music_time_label.text = format_text(progress_slider.value, 0.0)
+	if audio_player.has_stream_playback() && !is_changing && !audio_player.stream_paused \
+			&& audio_player.playing:
+		var position: float = position_in_part()
+		if _clip_length > 0.0 && position >= _clip_length:
+			# The short part has played, which counts as the song ending
+			audio_player.stop()
+			progress_slider.value = _clip_length
+			_on_finished()
+			return
+		if !Settings.hide_beginning || position > Settings.hide_for_seconds:
+			progress_slider.value = position
+		music_time_label.text = format_text(progress_slider.value, part_length())
+
+
+## How long the part of the song that plays is: the short part, or the whole song.
+func part_length() -> float:
+	if _clip_length > 0.0:
+		return _clip_length
+	return stream.get_length() if stream != null else 0.0
+
+
+## How far into the part that plays the song is, in seconds.
+func position_in_part() -> float:
+	return audio_player.get_playback_position() - _clip_start
+
+
+## Plays only part of [param song_object]'s song when Short songs is on and the song is
+## longer than the part. The part's start is picked once per song.
+func choose_part(song_object: SongObject, song: AudioStream) -> void:
+	_clip_start = 0.0
+	_clip_length = 0.0
+	var length: float = song.get_length()
+	if !Settings.short_songs || song_object == null || length <= Settings.short_song_seconds:
+		return
+	if song_object.short_start < 0.0:
+		var latest: float = minf(length * SHORT_START_MAX, length - Settings.short_song_seconds)
+		var earliest: float = minf(length * SHORT_START_MIN, latest)
+		song_object.short_start = randf_range(earliest, latest)
+	_clip_start = minf(song_object.short_start, length - Settings.short_song_seconds)
+	_clip_length = Settings.short_song_seconds
 
 
 ## Whether the play button shows pause, because a song is playing.
@@ -61,6 +107,14 @@ func _show_playing(playing: bool) -> void:
 		ButtonBounce.pop(play_button)
 
 
+# The progress bar and time show the length of the part that plays
+func _show_length() -> void:
+	var length: float = part_length()
+	progress_slider.max_value = round(length)
+	progress_slider.tick_count = 0 if Settings.hide_ticks else round(length / 60)
+	music_time_label.text = format_text(progress_slider.value, length)
+
+
 func format_time(time: float) -> String:
 	var total_seconds: int = roundi(time)
 	@warning_ignore("integer_division")
@@ -87,11 +141,7 @@ func play_or_pause() -> void:
 		song_finished = false
 		audio_player.seek(playback_position)
 		GameLog.debug("continue")
-		progress_slider.max_value = round(stream.get_length())
-		if Settings.hide_ticks:
-			progress_slider.tick_count = 0
-		else:
-			progress_slider.tick_count = round(stream.get_length() / 60)
+		_show_length()
 	else:
 		pause()
 
@@ -102,15 +152,15 @@ func pause() -> void:
 	_show_playing(false)
 
 
+## Plays the song, or its short part, from the beginning.
 func restart() -> void:
 	audio_player.stop()
 	audio_player.stream_paused = false
 	song_finished = false
 	progress_slider.value = 0
-	playback_position = audio_player.get_playback_position()
-	audio_player.seek(playback_position)
+	playback_position = _clip_start
 	_show_playing(true)
-	audio_player.play()
+	audio_player.play(_clip_start)
 
 
 func _on_finished() -> void:
@@ -127,16 +177,18 @@ func _on_drag_started() -> void:
 
 
 func _on_drag_ended(_changed: bool) -> void:
-	audio_player.seek(progress_slider.value)
-	playback_position = progress_slider.value
+	audio_player.seek(_clip_start + progress_slider.value)
+	playback_position = _clip_start + progress_slider.value
 	is_changing = false
 
 
+## Jumps [param new_position] seconds forward, or back when negative, staying within the
+## part that plays.
 func seek(new_position: float) -> void:
 	progress_slider.value += new_position
 	is_changing = true
-	audio_player.seek(progress_slider.value)
-	playback_position = progress_slider.value
+	audio_player.seek(_clip_start + progress_slider.value)
+	playback_position = _clip_start + progress_slider.value
 	is_changing = false
 
 
@@ -173,6 +225,9 @@ func play_sound(sound_name: AudioStream) -> void:
 
 
 func play_song(song: AudioStream) -> void:
+	# Sounds such as the winning song always play whole
+	_clip_start = 0.0
+	_clip_length = 0.0
 	audio_player.stream = song
 	audio_player.play()
 	sound_player.stop()
@@ -180,12 +235,7 @@ func play_song(song: AudioStream) -> void:
 	_show_playing(true)
 	stream = audio_player.stream
 	progress_slider.value = 0
-	progress_slider.max_value = round(stream.get_length())
-	if Settings.hide_ticks:
-		progress_slider.tick_count = 0
-	else:
-		progress_slider.tick_count = round(stream.get_length() / 60)
-	music_time_label.text = format_text(progress_slider.value, stream.get_length())
+	_show_length()
 
 
 ## Puts song [param song_object_no] in the player without playing it, e.g. the song that
@@ -196,12 +246,11 @@ func load_song_object(song_object_no: int) -> void:
 		return
 	audio_player.stream = song
 	stream = song
+	choose_part(Playlist.song_object_array[song_object_no], song)
 	song_finished = true
 	_show_playing(false)
 	progress_slider.value = 0
-	progress_slider.max_value = round(stream.get_length())
-	progress_slider.tick_count = 0 if Settings.hide_ticks else round(stream.get_length() / 60)
-	music_time_label.text = format_text(progress_slider.value, stream.get_length())
+	_show_length()
 
 
 func play_song_object(song_object_no: int) -> void:
@@ -214,20 +263,16 @@ func play_song_object(song_object_no: int) -> void:
 		audio_player.stop()
 		await get_tree().create_timer(0.5).timeout
 	audio_player.stream = song
+	choose_part(Playlist.song_object_array[song_object_no], song)
 	sound_player.stop()
-	audio_player.play()
+	audio_player.play(_clip_start)
 	song_finished = false
 	_show_playing(true)
 	# A new song makes the play button pop, also when it was already playing
 	ButtonBounce.pop(play_button)
 	stream = audio_player.stream
 	progress_slider.value = 0
-	progress_slider.max_value = round(stream.get_length())
-	if Settings.hide_ticks:
-		progress_slider.tick_count = 0
-	else:
-		progress_slider.tick_count = round(stream.get_length() / 60)
-	music_time_label.text = format_text(progress_slider.value, stream.get_length())
+	_show_length()
 	Playlist.set_currently_playing_song(song_object_no)
 	Playlist.unset_is_playing()
 	if !Settings.hide_next_track: