Add industrial metal soundtrack with seamless loop

- assets/audio/music/zombie_industrial_loop.ogg: 57.60 s Ogg Vorbis loop
  generated locally with YuE2-3B on the Tesla V100 (score written for this
  track; vocals removed with HTDemucs). The loop is spliced at the track's
  structural repeat with a 0.6 s crossfade: measured seam dip <= 1.6 dB.
- scripts/main.gd: "Music" AudioStreamPlayer at -11 dB, started at boot on
  desktop and on the start click on Web (browser autoplay policy). It forces
  stream.loop at runtime because the Web staging step re-imports the OGG with
  loop=false.
- tools/test_music.gd: soundtrack test (loop, length, volume, playback,
  still playing after player death).
- docs/MUSIC.md: source, loop construction, verification, licence.
- docs/WEB-DEPLOY.md: release ab6054111f5919a8 notes.

Initial commit of the project.

scripts/grenade.gd

@@ -0,0 +1,113 @@
+extends RigidBody3D
+signal detonated
+const FUSE_SECONDS := 4.0
+const BLAST_RADIUS := 5.0
+const MAX_DAMAGE := 160.0
+const Audio = preload("res://scripts/hl_audio.gd")
+const Blood = preload("res://scripts/blood_effects.gd")
+var age := 0.0
+var exploded := false
+var thrower: CharacterBody3D
+
+func _ready() -> void:
+	name = "Grenade"
+	add_to_group("grenades")
+	collision_layer = 16
+	collision_mask = 1 | 4
+	mass = 0.4
+	continuous_cd = true
+	linear_damp = 0.15
+	angular_damp = 0.6
+	var physics := PhysicsMaterial.new()
+	physics.bounce = 0.45
+	physics.friction = 0.65
+	physics_material_override = physics
+	var collider := CollisionShape3D.new()
+	var sphere := SphereShape3D.new()
+	sphere.radius = 0.12
+	collider.shape = sphere
+	add_child(collider)
+	var model: Node3D = load("res://assets/weapons/grenade.scn").instantiate()
+	add_child(model)
+	for area in model.find_children("*", "Area3D", true, false):
+		area.collision_layer = 0
+		area.collision_mask = 0
+	for mesh in model.find_children("*", "MeshInstance3D", true, false):
+		mesh.layers = 2
+	if is_instance_valid(thrower):
+		add_collision_exception_with(thrower)
+
+func _physics_process(delta: float) -> void:
+	age += delta
+	if age >= FUSE_SECONDS and not exploded:
+		exploded = true
+		call_deferred("_explode")
+
+func _explode() -> void:
+	freeze = true
+	collision_layer = 0
+	collision_mask = 0
+	hide()
+	var world := get_parent() as Node3D
+	var point := global_position
+	var targets := get_tree().get_nodes_in_group("live_zombies")
+	if is_instance_valid(thrower) and not thrower.death_started:
+		targets.append(thrower)
+	for target in targets:
+		if not is_instance_valid(target):
+			continue
+		var center: Vector3 = target.global_position + Vector3.UP * 0.9
+		var distance := point.distance_to(center)
+		if distance > BLAST_RADIUS:
+			continue
+		var query := PhysicsRayQueryParameters3D.create(point, center, 1)
+		if not get_world_3d().direct_space_state.intersect_ray(query).is_empty():
+			continue
+		var damage := MAX_DAMAGE * (1.0 - distance / BLAST_RADIUS)
+		if target == thrower:
+			target.take_damage(damage)
+		else:
+			Blood.impact(world, center, (center - point).normalized())
+			target.take_damage(damage, center.y)
+	_effect(world, point)
+	detonated.emit()
+	queue_free()
+
+func _effect(world: Node3D, point: Vector3) -> void:
+	var effect := Node3D.new()
+	effect.name = "GrenadeExplosion"
+	world.add_child(effect)
+	effect.global_position = point
+	Audio.play(effect, "weapons/explode%d.wav" % randi_range(3, 5), true, -7)
+	var light := OmniLight3D.new()
+	light.light_color = Color(1, 0.45, 0.08)
+	light.light_energy = 8
+	light.omni_range = 10
+	effect.add_child(light)
+	var tween := effect.create_tween()
+	tween.tween_property(light, "light_energy", 0.0, 0.3)
+	var particles := CPUParticles3D.new()
+	particles.amount = 60
+	particles.lifetime = 0.85
+	particles.one_shot = true
+	particles.explosiveness = 1
+	particles.direction = Vector3.UP
+	particles.spread = 180
+	particles.initial_velocity_min = 2
+	particles.initial_velocity_max = 6
+	particles.gravity = Vector3(0, -3, 0)
+	var mesh := SphereMesh.new()
+	mesh.radius = 0.06
+	mesh.height = 0.12
+	var material := StandardMaterial3D.new()
+	material.albedo_color = Color(1, 0.3, 0.05)
+	material.shading_mode = BaseMaterial3D.SHADING_MODE_UNSHADED
+	mesh.material = material
+	particles.mesh = mesh
+	var colors := Gradient.new()
+	colors.set_color(0, Color(1, 0.65, 0.1, 1))
+	colors.set_color(1, Color(0.15, 0.12, 0.1, 0))
+	particles.color_ramp = colors
+	effect.add_child(particles)
+	particles.emitting = true
+	world.get_tree().create_timer(4).timeout.connect(effect.queue_free)