svg-native: foundations

[rit3sh-x]

Summary

• Added the svg-native feature flag to blitz-dom and blitz-paint. It is additive and disabled by default.
• Root <svg> elements are recognized as SpecialElementData::SvgRoot and built into an SvgContext after layout, using the content-box as the viewport.
• SVG presentation attributes are now incorporated into the cascade at the PresHints level, while author CSS continues to take precedence.
• Added the CONSTRUCT_SVG damage bit and an SVG registry, allowing mutations to a single SVG fragment without affecting unrelated fragments or triggering full HTML box reconstruction.

Geometry/painting for shapes, <use>, hit-testing, and text are intentionally out of scope for this PR and will be addressed in follow-up PRs.

Reviewer Note

This PR is a breakdown of #693, which was a very extensive PR. The goal is to split the work into smaller, focused, and independently reviewable chunks.

[rit3sh-x]

Hey @nicoburns, I decided to convert #693 to draft and break it down into smaller, more focused, reviewable chunks. I think this will make the changes easier to review and lead to better discussions.

[nicoburns]

This seems like a good idea. This is much more reviewable than the previous PR! But perhaps we ought to start with a design document?

[rit3sh-x]

Hey @nicoburns, I put together a basic design that I used as a guide while working through the PR:

https://gist.github.com/rit3sh-x/c0aa5f7707009565af31d8f6590fd236

Is it sufficient? I can keep it as is. Otherwise, I can put together a more detailed design.