basic ECS spawner

addons/gecs/tests/systems/o_relationship_observer.gd

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+## Observer that watches relationships
+class_name O_RelationshipObserver
+extends Observer
+
+# Note: Observers don't currently have relationship_added/removed callbacks
+# This is a placeholder for when relationship observing is implemented
+var relationship_events: Array = []
+
+func watch() -> Resource:
+	return C_ObserverTest
+
+func match() -> QueryBuilder:
+	return q.with_all([C_ObserverTest])
+
+func reset() -> void:
+	relationship_events.clear()