Add cycle diagram shape layout

[vegapunk9527]

/claim #1578

Summary

• Add shape: cycle as a nested diagram layout shape.
• Position child nodes around a circle and route clockwise edges as cubic Bezier arcs trimmed at source/destination boxes.
• Add a txtar regression fixture covering 2, 3, 4, 5, and 6 node cycles for dagre and elk outputs.

Tests

• go test ./d2layouts/... ./d2graph/... ./d2target/...
• TA=1 go test ./e2etests -run 'TestE2E/txtar/cycle-diagram' -v
• go test ./e2etests -run 'TestE2E/txtar/cycle-diagram' -v
• ./ci/test.sh ./e2etests -run TestE2E/txtar/cycle-diagram -v
• TA=1 ./ci/test.sh ./e2etests -run TestE2E/txtar/cycle-diagram -v