◇ stability
Why a defect can keep its identity
Stable does not mean frozen. The mouth can wobble. The nearby flow can surge. The throat can ring for a while after something hits it. But small disturbances are treated more like elastic deformations than permanent damage: the defect can shed the extra energy and settle back toward its ordinary state.
The important part is that the basic identity of the defect is not a sliding scale. A mouth does not gradually become half an opposite mouth just because the surrounding flow gets messy. More water can enter. Less water can enter. The pattern can vibrate. But the opening remains the same kind of opening.
To actually change that identity, the model needs a bigger event: meeting a compatible opposite defect, leaving through a boundary, or changing the topology of the opening. That is what this site means when it says the defect is protected. In the Fluid Spacetime picture, this is part of the candidate explanation for why a particle-like object could keep the same identity over time.