Fix SVG intrinsic sizing for replaced elements

[nicoburns]

Summary

Implement CSS intrinsic sizing for SVG replaced elements instead of using usvg::Tree::size() as the intrinsic dimensions in every case.

SvgImageData tracks absolute root dimensions separately from the root viewBox aspect ratio. intrinsic_size() resolves both declared dimensions, derives a missing dimension from the intrinsic ratio, and applies the CSS default object size (300x150) for ratio-only SVGs.

This now uses usvg::Tree::intrinsic_dimensions() — a new upstream API (branch DioxusLabs/resvg#devin/1785858271-intrinsic-dimensions, proposed for linebender/resvg) exposing the declared root width/height/viewBox with percentages unresolved — instead of re-parsing the SVG XML with roxmltree. usvg is bumped to 0.48.1, whose viewBox-ratio missing-dimension fix (resvg#1045) also removes the previous inject-attribute-and-re-parse hack.

Temporary [patch.crates-io] entries point usvg/anyrender/anyrender_svg at git branches until the upstream resvg PR lands+releases and DioxusLabs/anyrender#78 is released.

WPT (css/css-flexbox, full local run)

• main: 713 PASS → branch: 721 PASS (+9 newly passing flex-aspect-ratio-img-*, −1 aspect-ratio-intrinsic-size-007.html)
• The 007 regression: the reference page uses <svg viewBox=...> with no width/height, which per SVG spec should default to width:100% but is now sized with the 300x150 default object size. Pre-existing in this PR before the usvg migration; needs separate handling of the root <svg> element's presentation-attribute defaults.

Test Plan

cargo test -p blitz-dom svg_tests, cargo test -p blitz-tests --test svg_background_size, cargo clippy -p blitz-dom, cargo fmt --check all pass.

Link to Devin session: https://app.devin.ai/sessions/1372da3cbba94b90a9c63cad94bdbdc1
Requested by: @nicoburns

WPT results

15 newly passing, 0 newly failing (net +15).

Full diff (15 changed tests)

+ Fail => Pass css/css-backgrounds/background-size/vector/tall--contain--height.html
+ Fail => Pass css/css-backgrounds/background-size/vector/tall--contain--width.html
+ Fail => Pass css/css-backgrounds/background-size/vector/wide--contain--height.html
+ Fail => Pass css/css-backgrounds/background-size/vector/wide--contain--width.html
+ Fail => Pass css/css-flexbox/flex-aspect-ratio-img-column-013.html
+ Fail => Pass css/css-flexbox/flex-aspect-ratio-img-column-014.html
+ Fail => Pass css/css-flexbox/flex-aspect-ratio-img-column-015.html
+ Fail => Pass css/css-flexbox/flex-aspect-ratio-img-column-018.html
+ Fail => Pass css/css-flexbox/flex-aspect-ratio-img-row-008.html
+ Fail => Pass css/css-flexbox/flex-aspect-ratio-img-row-009.html
+ Fail => Pass css/css-flexbox/flex-aspect-ratio-img-row-010.html
+ Fail => Pass css/css-flexbox/flex-aspect-ratio-img-row-011.html
+ Fail => Pass css/css-flexbox/flex-aspect-ratio-img-row-015.html
+ Fail => Pass css/css-sizing/aspect-ratio/replaced-element-039.html
+ Fail => Pass css/css-sizing/aspect-ratio/replaced-element-040.html

_{Generated by the WPT workflow.}

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