Function vertex_endpoint_gap 

pub fn vertex_endpoint_gap(vertex_point: Vec3, curve_end: Vec3) -> f64

|p_V − c_E(t_end)| — how far a curve end sits from the vertex point the topology says it meets.

The endpoint half of the edge → vertex propagation. On a freshly built carrier this is exactly zero for the edge that CLAIMED the vertex (the vertex was placed at that curve’s end) and non-zero for every later edge that reuses it — which is the whole reason it is worth measuring.