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.

tools/test_death_effects.gd

@@ -0,0 +1,52 @@
+extends SceneTree
+var world: Node3D
+func _initialize() -> void:
+	call_deferred("run_test")
+func run_test() -> void:
+	world = load("res://scenes/main.tscn").instantiate()
+	root.add_child(world)
+	Input.mouse_mode = Input.MOUSE_MODE_VISIBLE
+	await create_timer(2.4).timeout
+	var zombie: Node3D
+	for child in world.get_children():
+		if child.get_script() == world.ZombieScript:
+			zombie = child
+			break
+	assert(zombie != null)
+	world.player.hp = 10000
+	zombie.position = Vector3(0, 0, -3)
+	if zombie.entry_tween:
+		zombie.entry_tween.kill()
+	zombie.entering = false
+	var point := zombie.global_position + Vector3(0, 1.2, 0)
+	preload("res://scripts/blood_effects.gd").impact(world, point, Vector3(0, 0, -1))
+	zombie.take_damage(1000, point.y)
+	assert(zombie.dead and zombie.collision_layer == 0)
+	await create_timer(0.2).timeout
+	assert(get_nodes_in_group("blood_stains").size() == 1)
+	if DisplayServer.get_name() != "headless":
+		await RenderingServer.frame_post_draw
+		root.get_texture().get_image().save_png("/tmp/zombie-blood.png")
+	await create_timer(4.3).timeout
+	assert(is_instance_valid(zombie) and zombie.is_inside_tree())
+	assert(absf(zombie.position.y) < 0.01)
+	assert(get_nodes_in_group("zombie_corpses").has(zombie))
+	if DisplayServer.get_name() != "headless":
+		await RenderingServer.frame_post_draw
+		root.get_texture().get_image().save_png("/tmp/zombie-corpse.png")
+	world.player.take_damage(20000)
+	world.player.take_damage(12)
+	assert(world.player.death_started and world.player.hp == 0)
+	var ammo: int = world.player.ammo
+	world.player.shoot()
+	assert(world.player.ammo == ammo)
+	await create_timer(3).timeout
+	assert(world.player.camera.position.y < 0.25)
+	assert(world.player.death_overlay.color.a > 0.99)
+	if DisplayServer.get_name() != "headless":
+		await RenderingServer.frame_post_draw
+		root.get_texture().get_image().save_png("/tmp/zombie-death-fade.png")
+	print("PASS: persistent corpse, blood decal, single death, blocked shooting, camera fall, black fade")
+	world.queue_free()
+	await process_frame
+	quit()