feat(audio): add procedural gameplay sound foundation

Game/scripts/match_mode.gd

@@ -122,6 +122,7 @@ func _run_kickoff_countdown() -> void:
 	_set_frozen(true)
 	for count in range(KICKOFF_COUNTDOWN_SECONDS, 0, -1):
 		kickoff_countdown.emit(count)
+		AudioManager.play_countdown(count)
 		# process_always=false: if full-time fires mid-countdown (see
 		# _on_match_timer_timeout's get_tree().paused = true), this stalls
 		# harmlessly in lockstep with the pause instead of ticking a
@@ -132,6 +133,7 @@ func _run_kickoff_countdown() -> void:
 		if _match_over:
 			return
 	kickoff_countdown.emit(0)
+	AudioManager.play_countdown(0)
 	_set_frozen(false)