shape: add cycle diagram layout

[Zhaoziyuan2024]

Fixes #1578

Summary

• add shape: cycle as a legal diagram shape
• add a cycle layout that places direct children around a ring in declaration order
• route direct child edges inside cycle diagrams and support nested cycle extraction/reinjection
• add compiler validation for fixed positions and self-entering edges inside cycle diagrams

Tests

• go test ./d2layouts/...
• go test ./d2graph ./d2target
• go test ./d2compiler -run TestCompileCycleShape