d2svg: respect reduced motion for animated connections

[coyaSONG]

Summary

• add a prefers-reduced-motion: reduce override for animated SVG connections
• keep the override conditional so static diagrams and default animation behavior are unchanged
• cover the generated CSS directly and update every affected renderer/E2E snapshot
• add d2svg: respect reduced motion for animated connections #2778 to the next release changelog

Why

Animated connection paths receive their animation through an inline style. The reduced-motion rule therefore uses animation: none !important on .animated-connection so the operating-system accessibility preference reliably wins without changing the normal rendered output.

Validation

• ./make.sh lint with the repository-pinned Go 1.25.0 toolchain
• ./make.sh build with the repository-pinned Go 1.25.0 toolchain
• targeted unit, Dagre/ELK, sketch, dark-theme, bidirectional, and Markdown renderer tests
• ./make.sh test: all core packages and the complete e2etests package pass; the remaining e2etests-cli failures are unrelated external-fixture issues (changed remote icon bytes and a Playwright 1.52 driver URL returning 404)

Implementation and validation were assisted by OpenAI Codex; I reviewed the resulting diff and test output.

Fixes #1979