At every vertex (cell or waypoint) where two capsule segments meet, the union of the
rectangles and pad discs left a sharp inward V on the concave side. Canvas lineJoin
only smooths the convex side, so added an explicit tangent-arc region per concave
corner: intersect the inner offset lines to find the notch tip, place the arc center
on the angle bisector at radius padRadius, and fill the curved-triangle bounded by
the two tangent points and the arc. The added subpath unions into the existing fill.
Pan/zoom handlers redraw the cell holder canvas but skipped the busbar overlay, which was only drawn from updatePreview(). Cache the last draw args and expose redrawBusbarOverlay() so the overlay stays visible during wheel, drag, and touch interactions.
