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_grenade.gd

@@ -0,0 +1,70 @@
+extends SceneTree
+var detonation_time := -1
+func _initialize() -> void:
+	call_deferred("run_test")
+func run_test() -> void:
+	var world := Node3D.new()
+	root.add_child(world)
+	var room := Node3D.new()
+	room.set_script(load("res://scripts/room.gd"))
+	world.add_child(room)
+	var player := CharacterBody3D.new()
+	player.set_script(load("res://scripts/player.gd"))
+	player.position = Vector3(0, 0.1, 4)
+	world.add_child(player)
+	player.hp = 10000
+	Input.mouse_mode = Input.MOUSE_MODE_VISIBLE
+	await physics_frame
+	var grenade: RigidBody3D = player.throw_grenade()
+	assert(grenade != null)
+	assert(player.throw_grenade() == null)
+	assert(is_equal_approx(player.grenade_cooldown, 15))
+	assert(grenade.physics_material_override.bounce > 0)
+	grenade.detonated.connect(func(): detonation_time = Time.get_ticks_msec())
+	var started := Time.get_ticks_msec()
+	var start := grenade.position
+	await create_timer(0.5).timeout
+	assert(grenade.position.distance_to(start) > 1)
+	assert(grenade.linear_velocity.y < 3)
+	var targets: Array[Node3D] = []
+	for pos in [Vector3(0, 0, -2), Vector3(2, 0, 0), Vector3(0, 0, -7)]:
+		var target := Node3D.new()
+		target.set_script(load("res://tools/grenade_test_target.gd"))
+		target.position = pos
+		world.add_child(target)
+		targets.append(target)
+	var wall := StaticBody3D.new()
+	wall.position = Vector3(1, 1.5, 0)
+	var shape := CollisionShape3D.new()
+	var box := BoxShape3D.new()
+	box.size = Vector3(0.2, 3, 2)
+	shape.shape = box
+	wall.add_child(shape)
+	world.add_child(wall)
+	# Isolate explosion coverage from the projectile's random rotation/bounces.
+	grenade.freeze = true
+	grenade.global_position = Vector3(0, 0.3, 0)
+	await create_timer(3.15).timeout
+	assert(is_instance_valid(grenade) and not grenade.exploded)
+	await create_timer(0.6).timeout
+	assert(detonation_time >= 0)
+	var seconds := (detonation_time - started) / 1000.0
+	assert(seconds >= 3.8 and seconds <= 4.3)
+	assert(targets[0].damage_taken > 0)
+	assert(targets[1].damage_taken == 0) # behind a wall
+	assert(targets[2].damage_taken == 0) # beyond blast radius
+	assert(player.hp < 10000)
+	assert(player.throw_grenade() == null)
+	# Wait until the real cooldown expires, without changing the game timer.
+	await create_timer(player.grenade_cooldown + 0.1).timeout
+	assert(player.grenade_cooldown == 0)
+	assert(player.throw_grenade() != null)
+	var main = load("res://scripts/main.gd").new()
+	assert(main.zombies_for_wave(1) == 6)
+	assert(main.zombies_for_wave(2) == 8)
+	assert(main.zombies_for_wave(11) == 26)
+	main.free()
+	print("PASS: thrown physics/gravity, 4s fuse (", seconds, "), 15s cooldown, blast LOS/range/self-damage, wave growth beyond 24")
+	world.queue_free()
+	await process_frame
+	quit()