Fade out/in SFX bus during scene transitions

[wjt]

Currently we never adjust the SFX audio bus volume automatically, only under the user's manual control.

Unlike music (#637) I think it would actually be correct to fade SFX out and in along with the visual fade. For an example, go to Fray's End and start the musician's quest. I think it sounds bad that the ambient water/tree/white noise immediately plays at full volume; and that it cuts abruptly when you collect the thread and move to the ink combat scenes.

The simple option would be do this in transitions.gd, with e.g.

func _set_sfx_volume(volume_linear: float) -> void:
    AudioServer.set_bus_volume_linear(AudioServer.get_bus_index(&"SFX"), volume_linear)

# ...
var current_volume := AudioServer.get_bus_volume_linear(AudioServer.get_bus_index(&"SFX"))
tween_method(_set_sfx_volume, current_volume, 0.0)
# ...
tween_method(_set_sfx_volume, 0.0, current_volume)

This would need to be a parallel call on the same tweens.

Annoyingly Callable.bind binds from the right, not left, so can't be used.

XP Summary (total 300)

Subdomain	Skill	XP
Engineering: Gameplay & Systems Programming	Problem Solving	75
Engineering: Gameplay & Systems Programming	Creativity & Design Thinking	75
Engineering: Gameplay & Systems Programming	Technical Literacy	150