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/export_web_assets.gd

@@ -0,0 +1,17 @@
+extends SceneTree
+func _initialize() -> void:
+	for path in ["res://assets/weapons/v_9mmar", "res://assets/zombies/zombie_textured", "res://assets/weapons/grenade"]:
+		var scene: PackedScene = load(path + ".mdl")
+		assert(scene != null)
+		assert(ResourceSaver.save(scene, path + ".scn") == OK)
+		var node := scene.instantiate()
+		var textured := 0
+		for mesh in node.find_children("*", "MeshInstance3D", true, false):
+			for i in mesh.mesh.get_surface_count():
+				var material = mesh.mesh.surface_get_material(i)
+				assert(material is StandardMaterial3D and material.albedo_texture != null)
+				textured += 1
+		assert(textured > 0)
+		print("NATIVE SCENE VERIFIED ", path, " surfaces=", textured)
+		node.free()
+	quit()