feat(audio): drive engine tone from thrust

Game/scripts/audio_manager.gd

@@ -7,6 +7,7 @@ const SAMPLE_RATE := 44100
 const MAX_INTENSITY := 1.0
 
 var enabled := true
+var _engine_player: AudioStreamPlayer
 
 
 func bind_tree_buttons(root: Node) -> void:
@@ -47,6 +48,35 @@ func play_goal() -> void:
 	_play_tone(783.99, 0.30, 0.18)
 
 
+func set_engine_state(thrust: float, turbo: bool) -> void:
+	var amount := clamp_intensity(thrust)
+	if not enabled or amount <= 0.01:
+		stop_engine()
+		return
+	if _engine_player == null or not is_instance_valid(_engine_player):
+		_engine_player = AudioStreamPlayer.new()
+		_engine_player.stream = _engine_stream()
+		add_child(_engine_player)
+		_engine_player.play()
+	_engine_player.pitch_scale = engine_pitch(amount, turbo)
+	_engine_player.volume_db = linear_to_db(engine_volume(amount, turbo))
+
+
+func stop_engine() -> void:
+	if _engine_player != null and is_instance_valid(_engine_player):
+		_engine_player.stop()
+
+
+static func engine_pitch(thrust: float, turbo: bool) -> float:
+	var amount := clamp_intensity(thrust)
+	return 0.75 + amount * 0.55 + (0.30 if turbo and amount > 0.01 else 0.0)
+
+
+static func engine_volume(thrust: float, turbo: bool) -> float:
+	var amount := clamp_intensity(thrust)
+	return clampf(0.015 + amount * 0.045 + (0.025 if turbo and amount > 0.01 else 0.0), 0.0, 0.1)
+
+
 static func clamp_intensity(value: float) -> float:
 	if not is_finite(value):
 		return 0.0
@@ -72,6 +102,17 @@ func _play_tone(frequency: float, duration: float, volume: float) -> void:
 	player.play()
 
 
+func _engine_stream() -> AudioStreamWAV:
+	var stream := AudioStreamWAV.new()
+	stream.format = AudioStreamWAV.FORMAT_16_BITS
+	stream.mix_rate = SAMPLE_RATE
+	stream.stereo = false
+	stream.loop_mode = AudioStreamWAV.LOOP_FORWARD
+	stream.data = _tone_data(92.0, 1.0, 0.65)
+	stream.loop_end = SAMPLE_RATE
+	return stream
+
+
 func _tone_data(frequency: float, duration: float, volume: float) -> PackedByteArray:
 	var frames := maxi(1, int(duration * SAMPLE_RATE))
 	var data := PackedByteArray()