feat(audio): add procedural gameplay sound foundation

Game/scripts/audio_manager.gd

@@ -0,0 +1,71 @@
+extends Node
+
+# Dependency-free audio foundation. Authored assets can replace these tones
+# later without changing gameplay call sites or the multiplayer event flow.
+
+const SAMPLE_RATE := 44100
+const MAX_INTENSITY := 1.0
+
+var enabled := true
+
+
+func play_ui_click() -> void:
+	_play_tone(880.0, 0.045, 0.10)
+
+
+func play_countdown(count: int) -> void:
+	if count <= 0:
+		_play_tone(1046.5, 0.12, 0.18)
+	else:
+		_play_tone(countdown_frequency(count), 0.08, 0.14)
+
+
+func play_impact(intensity: float) -> void:
+	var amount := clamp_intensity(intensity)
+	if amount <= 0.0:
+		return
+	_play_tone(150.0 + 180.0 * amount, 0.06 + 0.08 * amount, 0.08 + 0.18 * amount)
+
+
+func play_goal() -> void:
+	_play_tone(523.25, 0.22, 0.22)
+	_play_tone(783.99, 0.30, 0.18)
+
+
+static func clamp_intensity(value: float) -> float:
+	if not is_finite(value):
+		return 0.0
+	return clampf(value, 0.0, MAX_INTENSITY)
+
+
+static func countdown_frequency(count: int) -> float:
+	return 440.0 + float(clampi(count, 1, 9)) * 55.0
+
+
+func _play_tone(frequency: float, duration: float, volume: float) -> void:
+	if not enabled or frequency <= 0.0 or duration <= 0.0 or volume <= 0.0:
+		return
+	var stream := AudioStreamWAV.new()
+	stream.format = AudioStreamWAV.FORMAT_16_BITS
+	stream.mix_rate = SAMPLE_RATE
+	stream.stereo = false
+	stream.data = _tone_data(frequency, duration, volume)
+	var player := AudioStreamPlayer.new()
+	player.stream = stream
+	add_child(player)
+	player.finished.connect(player.queue_free)
+	player.play()
+
+
+func _tone_data(frequency: float, duration: float, volume: float) -> PackedByteArray:
+	var frames := maxi(1, int(duration * SAMPLE_RATE))
+	var data := PackedByteArray()
+	data.resize(frames * 2)
+	for index in frames:
+		var envelope := minf(1.0, float(index) / 256.0) * minf(1.0, float(frames - index) / 1024.0)
+		var sample := int(sin(TAU * frequency * float(index) / SAMPLE_RATE) * volume * envelope * 32767.0)
+		if sample < 0:
+			sample += 65536
+		data[index * 2] = sample & 0xff
+		data[index * 2 + 1] = (sample >> 8) & 0xff
+	return data

Game/scripts/game_mode.gd

@@ -136,6 +136,7 @@ func _play_goal_celebration(scoring_team: int, conceding_team: int) -> void:
 	# real-time presentation delay between episodes.
 	if DisplayServer.get_name() == "headless" or not is_instance_valid(_camera_rig):
 		return
+	AudioManager.play_goal()
 	var goal_position := Vector3.ZERO
 	for goal in arena.get_goals():
 		if goal.team == conceding_team:

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)