Add audio feedback for combat events by introducing unit-specific sound
effects that play during attack animations. Each unit in civilization-rules.json
now has a battleSound property mapping to appropriate audio assets. Adds 10
new MP3 files (arrows, cannon, catapult, gunfire-classic/modern, helicopter,
missile, submarine, sword, tank) and integrates playback in CivilizationMapView's
combat lunge animation.
