Offset of an irregular closed spline no longer fails its error certificate; offset fit nodes stay private (#1009)

Fixes #971. At the node limit the offset certificate splits the fitted curve where it meets the exact offset midpoint, so curves with different parameter speeds certify within the same 0.002 mm tolerance. Associated offset fit nodes are hidden from display, picking, box selection and snapping until Break Association.

Thanks @Karl-VonRichter.

ROADMAP.md

@@ -270,8 +270,12 @@ PCB / Package, Render, Animation, Simulation, Manufacture/CAM, Drawings.
   Sweeps along 3D sketch curves follow their polyline (smooth walls when the path is smooth).
 - Extend intersects exact ellipses and rational/high-degree splines without moving the line or
   circular support. Associative free-form offsets have a whole-span 0.002 mm normal-distance
-  certificate and refine within bounded node limits while retaining existing IDs. Singular,
-  self-intersecting and ambiguous curves refuse atomically; mixed junctions and nonsmooth
+  certificate and refine within bounded node limits while retaining existing IDs.
+  At the node limit, geometric certificate subdivisions accommodate unequal spline parameter
+  speeds without changing stored nodes or relaxing the distance tolerance (#971).
+  Associated offset fit nodes stay hidden from display, picking and snapping; dragging the
+  editable source updates the derived boundary live. Break Association restores point editing.
+  Singular, self-intersecting and ambiguous curves refuse atomically; mixed junctions and nonsmooth
   closed natural-spline seams remain limitations. New explicitly closed fit splines and closed
   offset outputs use periodic C2 interpolation, supporting repeated certified offsets; older
   natural splines are preserved. Clicking the starting fit point deliberately closes a new loop.