[support] weak var

[muukii]

  @Test
  func weakRef() {
    
    let state = MyState.init()
    
    let result = state.tracking {
      _ = state.weak_ref
    }
    
    #expect(
      result.graph.prettyPrint() == """
        StateStructTests.MyState {
          weak_ref-(1)
        }
        """
    )
  }