Native inline SVG foundations behind the svg-native feature - #1541

Milestone 1 (foundations) of native SVG rendering. Each SVG backend is now its own cargo feature: svg-usvg is the existing path (inline <svg> serialized and re-parsed by usvg, plus <img src="x.svg"> and CSS background SVGs), with svg kept as an alias; svg-native keeps an inline <svg> subtree in the DOM as live nodes, stylo styles them, and blitz-paint draws the shapes from their computed styles. The native feature compiles without usvg (SvgIntrinsicDimensions has its own SvgViewBox/SvgTransform types, and CI clippy-checks blitz-dom and blitz-paint with only svg-native). When both features are compiled, DocumentConfig { native_svg: Some(true), .. } picks the native renderer per document; the default is usvg, so nothing changes for anyone until the flag is set. With one backend compiled the flag is ignored. <img>/background SVGs stay on usvg either way. dioxus-native forwards svg-native; the WPT runner and the screenshot example compile both backends and switch with BLITZ_NATIVE_SVG=1. Nothing about the usvg workaround PRs (#587, #1267, #1320, #1266, #1510) is touched.

What the native path does:

• is_html_element / is_svg_element / is_html_element_in_html_document now check the element namespace (ns!(html) / ns!(svg)) instead of always answering "HTML". The UA sheet gets svg| rules for overflow on inner viewports, transform-origin, and non-rendered title/desc/metadata/script/style. (The @namespace svg declaration has to sit with the other @namespace rules at the top of the sheet: declared after a style rule it is invalid and every svg| rule silently drops, which is what the first version of this PR did.) Resource containers (defs, gradients, clipPath, mask, marker, pattern, symbol, filter) are deliberately not display: none: stylo wouldn't style their children, and later milestones need those computed styles. The painter skips them by tag instead.
• Presentation attributes are turned into presentation hints with stylo's own property parser (stylo/svg_attrs.rs), so the cascade order is right for free (attribute < author CSS). Each attribute is parsed into one stack-allocated SourcePropertyDeclaration that is reused across the element's attributes and drained into its declaration block, so there is no per-attribute allocation. Geometry attributes are mapped only on the elements they belong to (cx/cy on circle/ellipse, r on circle, rx/ry on rect/ellipse, x/y/width/height on rect/image/foreignObject/svg, d on path); elsewhere they stay plain attributes. d goes through SVGPathData::parse_bytes, so a malformed suffix keeps the valid prefix as SVG requires. transform="..." is composed into a single matrix(...) declaration. Out-of-range values (negative rx, r) are dropped so the property stays auto/initial rather than clamped.
• The <svg> root is a replaced box with SpecialElementData::SvgRoot(SvgIntrinsicDimensions) computed from width/height/viewBox/preserveAspectRatio (the same struct the usvg path uses, now built from attributes instead of an xmltree). No boxes are constructed for descendants.
• Damage: a native <svg> root is a boundary. Descendant damage is reduced to REPAINT when it reaches the root (the HTML page is never relaid out because a circle moved); only the root's own damage (sizing attributes, style) is layout damage for the ancestors. In restyle_for_attribute_change, only the root's width/height/viewBox/preserveAspectRatio are layout-read in native mode; other SVG attribute changes mark repaint damage. The old "any SVG attribute reconstructs the whole SVG" behaviour is kept for usvg mode.
• blitz-paint/src/render/svg.rs walks the live subtree and paints rect/circle/ellipse/line/polyline/polygon/path, g/a, and nested <svg> viewports, honouring transform, fill/fill-rule/fill-opacity, stroke with width/caps/joins/dasharray/offset/opacity, group opacity, visibility/display, currentColor, and overflow clipping on inner viewports.

Known gaps in what is here: transform-origin percentages and keywords on SVG content resolve against an empty reference box (there is no transform-box support yet), so transform-origin: center rotates about the user-space origin while lengths work; presentation-hint URLs (fill="url(#g)") are parsed against a placeholder base URL until paint servers land; <switch> paints its first child without evaluating conditional attributes. The namespace-aware is_svg_element/is_html_element and the shared UA rules (svg|title etc. hidden, transform-origin: 0 0 on SVG content) apply to computed styles in usvg mode too; CI's WPT diff shows no rendering change from that.

Not in this PR (later milestones; these render as nothing in native mode, so leave the flag off if you need them): <use>, paint servers (gradients/patterns), markers, clip-path/mask on SVG content, filters, <text>, <image>, <foreignObject> content, hit testing of SVG shapes and pathLength. There is no per-<svg> fallback to usvg (decided against in the scope discussion).

WPT svg/** (1520 tests, 15,881 subtests, PR head): main (usvg) 6036 subtests pass, this PR with usvg 6045, this PR native 6281. On the shapes/styling/painting/render/coordinate-systems/geometry/struct-reftests subset (561 tests, 2115 subtests): 944 / 950 / 1180, native +269 / −39 subtests vs usvg on the same build. The native losses (35 tests) are in features listed above (markers, <use>, text, clip-path/mask, pathLength) plus svg/styling/nested-svg-sizing-* (nested <svg> width/height given as auto, calc(), em/rem, % or sizing keywords: the native painter only resolves plain lengths and percentages for nested viewports). CI runs the default (usvg) mode, so the diff below is the usvg-mode change; a second native pass over the SVG directories in CI is a follow-up.

The usvg backend's rendering is unchanged: presentation hints are only synthesized for documents that render SVG natively (otherwise the outer <svg>'s transform attribute would apply twice), and the SVG UA sheet's svg:not(:root) { overflow: hidden } lives in a separate NATIVE_SVG_CSS UA sheet added only to those documents (as overflow: clip, so the replaced <svg> is not a scroll container and keeps its automatic minimum size as a flex item), since usvg transforms the outer <svg>'s content rather than its CSS box. Three svg/animations SMIL tests are quarantined (animation-events): now that type selectors match SVG elements, they wait for endEvent instead of throwing.

Tests: tests/blitz-tests/tests/native_svg.rs (both features compiled, switched per document) covers toggle/fallback behaviour, namespace matching, presentation attributes (incl. element-specific geometry and partial d), transforms, intrinsic sizing, currentColor, group opacity, nested-viewport clipping, resource containers (styled but not painted) and repaint-on-attribute-change; layout/damage.rs has unit tests for the root boundary. Also includes the needless_bool fixes the current stable clippy raises on main. cargo fmt, the clippy invocations (incl. the two new svg-native-only ones), cargo test --workspace, cargo doc and cargo deny ... check licenses pass.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/5b57528a8ccb4275ab16b1320f65bea5
Link to Bot: https://dioxus.staging.devinenterprise.com/org/dioxus-engineering/bots/3904245e951a4a7fa4631b37b308d89b
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/5b57528a8ccb4275ab16b1320f65bea5?variant=devin-insiders

Painting opened this PR for Nico

Implemented in Build native SVG foundations in blitz-dom · Review with Devin

WPT results

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

+ css           | +6 (+6 / -0) | 59.69% -> 59.70% (+0.00%) | 119623 -> 119629 / 200400
+   css-display | +1 (+1 / -0) | 53.99% -> 54.14% (+0.15%) |    372 ->    373 /    689
+   selectors   | +5 (+5 / -0) | 48.65% -> 48.75% (+0.10%) |   2528 ->   2533 /   5196
+     old-tests | +5 (+5 / -0) | 90.25% -> 92.06% (+1.81%) |    250 ->    255 /    277
+ svg           | +9 (+9 / -0) | 38.01% -> 38.06% (+0.06%) |   6036 ->   6045 /  15881
+   animations  | +1 (+1 / -0) |  4.32% ->  4.59% (+0.27%) |     16 ->     17 /    370
+   geometry    | +4 (+4 / -0) | 46.28% -> 46.91% (+0.63%) |    292 ->    296 /    631
+     parsing   | +4 (+4 / -0) | 48.38% -> 49.68% (+1.30%) |    149 ->    153 /    308
+   interact    | +2 (+2 / -0) | 47.42% -> 47.44% (+0.02%) |   4398 ->   4400 /   9275
+     scripted  | +2 (+2 / -0) | 47.55% -> 47.57% (+0.02%) |   4387 ->   4389 /   9227
+   styling     | +2 (+2 / -0) | 32.06% -> 32.48% (+0.42%) |    151 ->    153 /    471

+ FAIL => PASS    [1/1]  +1  css/css-display/display-contents-svg-elements.html
+ FAIL => PASS    [1/1]  +1  css/selectors/old-tests/css3-modsel-173a.xml
+ FAIL => PASS    [1/1]  +1  css/selectors/old-tests/css3-modsel-173b.xml
+ FAIL => PASS    [1/1]  +1  css/selectors/old-tests/css3-modsel-174a.xml
+ FAIL => PASS    [1/1]  +1  css/selectors/old-tests/css3-modsel-92.xml
+ FAIL => PASS    [1/1]  +1  css/selectors/old-tests/css3-modsel-93.xml
! FAIL => SKIP    [0/1]  +0  svg/animations/beginelement-instance-time-1.html
+ FAIL => PASS    [2/2]  +1  svg/animations/historical.html
! FAIL => SKIP    [0/1]  +0  svg/animations/scripted/animatetransform-type-missing-value-default.html
! FAIL => SKIP    [0/1]  +0  svg/animations/scripted/keypoints-attribute-trailing-semi.html
+ FAIL => FAIL  [17/34]  +2  svg/geometry/parsing/height-computed.svg
+ FAIL => FAIL  [17/34]  +2  svg/geometry/parsing/width-computed.svg
+ FAIL => FAIL  [23/39]  +2  svg/interact/scripted/tabindex-focus-flag.svg
+ FAIL => PASS    [8/8]  +2  svg/styling/css-selectors-case-sensitivity.html

_{Generated by the WPT workflow.}