chore(client): add gdUnit4 test framework addon

Vendor gdUnit4 for Godot 4 client testing (D-030).

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

client/addons/gdUnit4/src/monitor/GdUnitMonitor.gd

@@ -0,0 +1,24 @@
+# GdUnit Monitoring Base Class
+class_name GdUnitMonitor
+extends RefCounted
+
+var _id :String
+
+# constructs new Monitor with given id
+func _init(p_id :String) -> void:
+	_id = p_id
+
+
+# Returns the id of the monitor to uniqe identify
+func id() -> String:
+	return _id
+
+
+# starts monitoring
+func start() -> void:
+	pass
+
+
+# stops monitoring
+func stop() -> void:
+	pass