basic ECS spawner

addons/gecs/tests/systems/o_velocity_observer.gd

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+## Observer approach for velocity-based movement (reactive)
+class_name O_VelocityObserver
+extends Observer
+
+var process_count: int = 0
+
+func watch() -> Resource:
+	return C_TestVelocity
+
+func match() -> QueryBuilder:
+	return q.with_all([C_TestPosition, C_TestVelocity])
+
+func on_component_changed(
+	entity: Entity, component: Resource, property: String, old_value: Variant, new_value: Variant
+) -> void:
+	if property == "velocity":
+		process_count += 1
+		# React to velocity changes by updating position
+		var pos = entity.get_component(C_TestPosition)
+		if pos:
+			# Example: Could apply immediate velocity change
+			# In reality, observers are better for reactions than continuous updates
+			pass
+
+func on_component_added(entity: Entity, component: Resource) -> void:
+	process_count += 1
+
+func reset_count():
+	process_count = 0