Fix iOS/Android audio delay and improve sound layering

[john-doherty]

On iOS Safari, AudioContext is suspended at page load and can only be resumed on a user gesture. The previous code called audioCtx.resume() inside playSound() - but since resume() is async, the first sound after a gesture was delayed by 50–200ms. iOS also re-suspends audio after phone calls, Siri, or app switching, and there was no recovery for that.

Additionally, playSound always stopped any existing instance before playing, so rapid collisions (bricks, bullets) would cut each other off instead of overlapping.

Changes

• _unlockAudio() - one-shot listener on touchstart/pointerdown/mousedown/keydown that calls audioCtx.resume() and plays a silent 1-sample buffer to warm the iOS audio graph, then removes itself
• visibilitychange + focus listeners - resume AudioContext when the page regains visibility, covering iOS interruptions
• playSound reworked - short SFX now allow up to 3 overlapping instances; the 4th evicts the oldest. Pass { loop: true } or { restart: true } to get the old single-instance behaviour. Fallback resume() kept as a safety net
• README updated - the previous description ("calling playSound twice restarts it") was incorrect for SFX; now correctly describes the overlap-up-to-3 behaviour
• Sound tests expanded - 4 new behavioural tests covering rapid play, cap eviction, loop restart, and stop after multiple plays