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/spr_importer.gd

@@ -0,0 +1,103 @@
+@tool
+extends EditorImportPlugin
+
+
+func _get_importer_name() -> String:
+	return "goldsrc.spr"
+
+
+func _get_visible_name() -> String:
+	return "GoldSrc SPR"
+
+
+func _get_recognized_extensions() -> PackedStringArray:
+	return PackedStringArray(["spr"])
+
+
+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"
+
+
+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": "animation_fps",
+			"default_value": 10.0,
+			"property_hint": PROPERTY_HINT_RANGE,
+			"hint_string": "1.0,60.0,0.5",
+		},
+		{
+			"name": "loop",
+			"default_value": true,
+		},
+	]
+
+
+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 fps: float = options.get("animation_fps", 10.0)
+	var loop: bool = options.get("loop", true)
+
+	var spr := GoldSrcSPR.new()
+	var err = spr.load_spr(source_file)
+	if err != OK:
+		push_error("SPR Importer: Failed to load '%s': %s" % [source_file, error_string(err)])
+		return err
+
+	var frame_count = spr.get_frame_count()
+	if frame_count == 0:
+		push_error("SPR Importer: No frames in '%s'" % source_file)
+		return ERR_INVALID_DATA
+
+	var sprite = spr.build_scene()
+	if not sprite:
+		push_error("SPR Importer: build_scene() failed for '%s'" % source_file)
+		return ERR_CANT_CREATE
+
+	var sprite_name := source_file.get_file().get_basename()
+	if sprite_name.is_empty():
+		sprite_name = "sprite_root"
+	sprite.name = sprite_name
+
+	# Bake import options into the SpriteAnimationPlayer child
+	var sap = sprite.get_node_or_null(^"SpriteAnimationPlayer")
+	if sap:
+		sap.fps = fps
+		sap.loop_animation = loop
+
+	var scene := PackedScene.new()
+	var pack_err := scene.pack(sprite)
+	sprite.free()
+	if pack_err != OK:
+		push_error("SPR Importer: Failed to pack scene for '%s': %s" % [source_file, error_string(pack_err)])
+		return pack_err
+
+	return ResourceSaver.save(scene, save_path + "." + _get_save_extension())