Add cycle diagram shape

[lymeow233-wq]

/claim #1578

Summary

• add shape: cycle as a nested diagram layout with circular arcs clipped to child shape bounds
• add compiler validation for cycle diagrams so fixed positions and self-entering cycle edges fail clearly
• harden Windows golden/e2e coverage by accepting only newline-only golden diffs, normalizing fixture CRLF at read boundaries, and making import/watch/path tests deterministic

Why

This implements the cycle shape from #1578 and keeps the edge arcs circular while trimming endpoints to object borders. It also closes Windows test gaps that were causing full-package golden comparisons to fail noisily.

Validation

• CI=1 go test --timeout=30m ./... -count=1
• CI=1 go test --timeout=30m ./e2etests -count=1
• CI=1 go test --timeout=30m ./e2etests-cli -count=1
• go test ./d2layouts/d2cycle ./internal/testdiff ./d2compiler ./d2exporter -count=1
• go test ./e2etests -run TestE2E/txtar/cycle-diagram -count=1
• git -c core.autocrlf=false diff --check origin/master..HEAD
• confirmed no *.got.json, *.got.svg, or *.got.txt artifacts remain