fix(edges): align arrow markers with the stroke they terminate

[Ken-vdE]

Arrowheads pointed somewhere the edge did not, and the stroke poked out of the triangle's side. Two independent causes, both about the path tangent an orient="auto" SVG marker reads at its vertex.

1. getBezierPath whips into the target handle's normal over the final few pixels: on a real template edge the tangent turns 42 degrees within the last 12px. The marker takes the exact endpoint tangent, and refX puts the tip on the endpoint, so the whole triangle sits over curve running a different way. Reserve a straight lead as long as the arrow marker at both endpoints, so the stroke stops where the arrowhead begins and both share one direction. The leads are tangent-continuous with the curve because getBezierPath already leaves and enters along the same handle normals. Skipped on edges shorter than 48px.

2. buildCurvedPath anchors smooth splines by duplicating the first and last point, which makes d3 emit zero-length commands and cubics whose control points collapse onto the endpoint. The tangent is then zero and browsers fall back to 0 degrees, so ELK-routed and manual-waypoint edges pointed right no matter which way they ran. normalizeMarkerTangents strips the degenerate commands and restores a readable tangent; a fully collapsed cubic traces its own chord, so it becomes a lineTo. Endpoints are untouched and basis output now matches d3's un-anchored tail.

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After