Trim cycle diagram arcs to object bounds

[mgalavai]

Summary

• route cycle diagram edges as true cubic Bezier circle segments instead of sampled pseudo-curves
• trim arc start/end angles at source and destination object boxes so edges begin/end at bounds
• regenerate cycle-diagram txtar expected JSON/SVG fixtures

Status

Withdrawn from bounty consideration.

Test plan

• go test ./d2layouts/d2cycle ./d2renderers/d2svg
• TA=1 ./ci/test.sh ./e2etests -run TestE2E/txtar/cycle-diagram -v

[nagiexplorer88]

I think this still misses the border-clipping part of #1578 for non-rectangular cycle children.

createCircularArc trims only against edge.Src.Box and edge.Dst.Box via trimStartAngle / trimEndAngle, then assigns edge.Route = cubicArcRoute(...). There is no follow-up through TraceToShape / TraceToShapeBorder, and the regenerated cycle-diagram fixture appears to cover only default rectangle nodes.

That means a cycle child with shape: circle, oval, or hexagon can stop on the bounding box instead of the visible shape outline, while the bounty asks for the paths to start/stop at shape bounds. Adding one non-rectangle cycle fixture and clipping the first/last route points to the actual shape border would cover that case.

[mgalavai]

Withdrawing this PR from bounty consideration.