[lumatic2]

fixes #23981

Since #17988, traverseNodes walks into <template> content, so a <link rel="stylesheet"> inside a template hits the same branch as a link in the document. In build the tag is removed and its CSS is bundled into the <head> stylesheet, so a declarative shadow DOM template loses its styles. Dev leaves the link as is.

This skips the CSS import for links inside a template. They go through processAssetUrl like links with media or disabled (#6748), so the href is rewritten and the tag stays in the template. The file is emitted as a plain asset, which means CSS plugins don't run on it and a small file gets inlined as a data: URL.

I added a <template shadowrootmode="open"> with a stylesheet link to the assets playground. The tests check the color inside the shadow root and that the built HTML keeps the link in the template. Both fail without the fix.

I used Claude Code to trace the code path and draft the change and tests. I reviewed it myself: I read the link branch in html.ts and ran the new tests with and without the fix.