chore(tooling): install GUT v9.6.0 test framework

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

addons/gut/double_templates/double_data_template.txt

@@ -0,0 +1,22 @@
+var __gutdbl_values = {
+	thepath = '{path}',
+	subpath = '{subpath}',
+	stubber = {stubber_id},
+	spy = {spy_id},
+	gut = {gut_id},
+	singleton_name = '{singleton_name}',
+	singleton = {singleton_id},
+	is_partial = {is_partial},
+	doubled_methods = {doubled_methods},
+}
+var __gutdbl = load('res://addons/gut/double_tools.gd').new(self)
+
+# Here so other things can check for a method to know if this is a double.
+func __gutdbl_check_method__():
+	pass
+
+# Cleanup called by GUT after tests have finished.  Important for RefCounted
+# objects.  Nodes are freed, and won't have this method called on them.
+func __gutdbl_done():
+	__gutdbl = null
+	__gutdbl_values.clear()