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.

addons/goldsrc/importer/mdl_importer.gd

@@ -0,0 +1,112 @@
+@tool
+extends EditorImportPlugin
+
+
+func _get_importer_name() -> String:
+	return "goldsrc.mdl"
+
+
+func _get_visible_name() -> String:
+	return "GoldSrc MDL"
+
+
+func _get_recognized_extensions() -> PackedStringArray:
+	return PackedStringArray(["mdl"])
+
+
+func _get_save_extension() -> String:
+	return "scn"
+
+
+func _get_resource_type() -> String:
+	return "PackedScene"
+
+
+func _get_preset_count() -> int:
+	return 1
+
+
+func _get_preset_name(preset_index: int) -> String:
+	return "Default"
+
+
+## What shape of scene this importer builds. 1: meshes merged into one MeshInstance3D per
+## bodypart. 2: triangle corners welded back into shared vertices.
+##
+## Godot records it in each .import file but does not act on a bump — neither --import nor an
+## editor session reimports on its own (checked on 4.7). Changing the built scene means clearing
+## .godot/imported/*.mdl-* by hand; the number is here so it is visible which build a cache came
+## from.
+func _get_format_version() -> int:
+	return 2
+
+
+func _get_priority() -> float:
+	return 1.0
+
+
+func _get_import_order() -> int:
+	return 0
+
+
+func _can_import_threaded() -> bool:
+	return false
+
+
+func _get_import_options(_path: String, _preset_index: int) -> Array[Dictionary]:
+	return [
+		{
+			"name": "scale_factor",
+			"default_value": 0.025,
+			"property_hint": PROPERTY_HINT_RANGE,
+			"hint_string": "0.001,1.0,0.001",
+		},
+	]
+
+
+func _get_option_visibility(_path: String, _option_name: StringName, _options: Dictionary) -> bool:
+	return true
+
+
+func _import(source_file: String, save_path: String, options: Dictionary,
+		_platform_variants: Array[String], _gen_files: Array[String]) -> Error:
+	var scale_factor: float = options.get("scale_factor", 0.025)
+
+	var mdl := GoldSrcMDL.new()
+	mdl.set_scale_factor(scale_factor)
+
+	var err = mdl.load_mdl(source_file)
+	if err != OK:
+		push_error("MDL Importer: Failed to load '%s': %s" % [source_file, error_string(err)])
+		return err
+
+	mdl.build_model()
+
+	# Reparent children to a plain Node3D so the saved scene is GDExtension-independent
+	var root := Node3D.new()
+	var root_name := source_file.get_file().get_basename()
+	if root_name.is_empty():
+		root_name = "mdl_root"
+	root.name = root_name
+	# GoldSrc models face -Z after axis swizzle; rotate to match GoldSrc game orientation
+	root.rotation_degrees.y = 90.0
+
+	var children := mdl.get_children()
+	for child in children:
+		mdl.remove_child(child)
+		root.add_child(child)
+	_set_owner_recursive(root, root)
+
+	var scene := PackedScene.new()
+	var pack_err := scene.pack(root)
+	if pack_err != OK:
+		push_error("MDL Importer: Failed to pack scene: %s" % error_string(pack_err))
+		return pack_err
+
+	return ResourceSaver.save(scene, save_path + "." + _get_save_extension())
+
+
+func _set_owner_recursive(node: Node, owner: Node) -> void:
+	for child in node.get_children():
+		child.owner = owner
+		_set_owner_recursive(child, owner)