Audio buses and positional audio

Audio buses are a tree of gains a sound plays through, every chain ending at master. One volume moves everything on a bus, and a listener gives positional audio.

• Buses are declared in project.toml. A sound's gain is its own volume times every bus's up to the root, so audio::set_bus_volume("music", 0.3) moves every piece of music at once and nothing else. master exists whether or not the project declares it.
• A bus does not own a sound. Routing is per playback: one file is a footstep on sfx and a menu click on ui.
• audio::play_event(name, options) plays a named sound: the next of its variations in turn, at its own volume and pitch, through its own bus. A position in the options places it.
• A listener node is the ears, or audio::set_listener puts them somewhere by hand. set_emitter_position moves what a handle plays from, and distance_gain says what the distance is costing.

[audio.buses]
sfx = { volume = 0.9 }
music = { volume = 0.6 }
ui = { volume = 1.0, parent = "sfx" }

There are no effects and no sends. The audio module is in the script modules reference.