d2svg: render underline style on markdown labels

[SR0725]

Summary

Markdown labels kept style.underline in the target shape, but the SVG renderer did not apply it inside the foreignObject. This adds text-decoration: underline to the Markdown container only when that style is enabled.

The new Dagre e2e fixture covers the rendered HTML and board data for an underlined Markdown label.

Validation

• ./make.sh fmt
• ./make.sh lint
• ./make.sh test
• ./make.sh race
• ./make.sh build

Result

The change only affects Markdown shapes with style.underline: true; other Markdown and plain-text labels keep their existing rendering.

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

Fixes #1959

[SR0725]

Current master now covers the underline case through #2866. The native Markdown renderer passes targetShape.Underline into markdown.render, and d2renderers/d2svg/markdown_native_test.go asserts the resulting text-decoration="underline". That supersedes this foreignObject patch, so I’m closing it instead of rebasing a redundant implementation.