debugSfxPoolFreeCount property
How many SFX players _sfxPool is currently holding onto, idle,
ready for the next playSfx call — bounded by sfxPoolCapacity even
right after a burst wider than that (the excess gets disposed on
release instead of retained, see _sfxPool's doc comment).
Implementation
@visibleForTesting
int get debugSfxPoolFreeCount => _sfxPool.freeCount;