refactored sound into internal closure and improved tests

[john-doherty]

All audio state, functions, and event listeners are now grouped into a single var audio = (function(){ ... })() closure, replacing ~7 scattered module-level variables and ~10 functions across the file. No behaviour changes — same iOS unlock flow, stall detection, and pending sound queue. Engine code consumes it via audio.load(), audio.play(), audio.stop(), audio.stopAll(), and audio.checkStall(). Public punter.playSound/punter.stopSound remain unchanged.

Also adds 7 new tests covering pre-setup guards, buffer source creation counts, loop/once/restart tracking behaviour, and scene-transition sound stopping, using AudioContext.prototype spies rather than exposing any internal state.