Fix render engine: CSS grid, cascade correctness, SVG, inline flow

The render layer had no CSS Grid support, so Vector 2022's page layout
  (three nested grids) collapsed to a vertical stack with the article
  landing tens of thousands of pixels down the page. Wikipedia rendered
  as a near-blank page and Hacker News only worked through per-site
  hardcoded CSS overrides.

  Removed the HN-specific hardcoding and fixed the general engine instead:

  - CSS Grid layout (grid-template-areas/columns/rows via taffy's grid mode)
  - Indexed, specificity-correct cascade in a new css.rs, replacing the
    old test-every-selector-against-every-node scan. Cascade time on
    Wikipedia went from 9.5s to under 200ms.
  - Ancestor bloom-filter fast-rejection for selector matching in
    obscura-dom, needed to make the indexed cascade actually fast on
    descendant combinators.
  - :link/:visited pseudo-classes were previously unparseable, so any
    rule using them (a:link{color:...}, extremely common) silently
    dropped the whole rule from the cascade.
  - overflow:hidden clipping, fixing the pervasive visually-hidden-but-
    accessible pattern used for skip links and screen-reader-only labels.
  - @media parser robustness: it did a literal substring search for
    max-width: and silently dropped the constraint on max-width : 750px
    (space before colon) or max-width:calc(750px - 1px), treating
    mobile-only or narrow-viewport rules as always active. The calc()
    case was hiding Wikipedia's TOC sidebar via a rule meant only for
    narrow screens.
  - Table auto-layout approximation: the trailing auto-width td in a row
    absorbs leftover space, so a width:100% table's unsized last column
    reaches the right edge instead of shrinking to content.
  - Inline formatting context approximation: a block holding inline-level
    children (a paragraph or list item with links) is promoted to a
    wrapping flex row so text and links read as continuous prose instead
    of one fragment per line.
  - Whitespace-only text nodes no longer get a full line-height box. They
    used to stack up as blank rows, turning Wikipedia's collapsed TOC
    subsections and the page header into hundreds of pixels of dead space.
  - SVG rendering via resvg/usvg, plus resolving relative image URLs
    against the page's base URL.
  - background-image support including background-size/background-position
    for correctly sized and positioned icons.
  - Per-screenshot image fetch cache.
  - Fixed the layout-only (no paint feature) build, which didn't compile
    due to an unconditional call into paint-gated code.

crates/obscura-render/Cargo.toml

@@ -10,7 +10,7 @@ description = "Optional scoped render layer for Obscura: layout (taffy) and, wit
 default = []
 # Rasterize the laid-out tree to a pixmap (tiny-skia) with text (ab_glyph) and images.
 # Off by default; the layout core (getBoundingClientRect etc.) works without it.
-paint = ["dep:tiny-skia", "dep:ab_glyph", "dep:image", "dep:ureq", "dep:base64"]
+paint = ["dep:tiny-skia", "dep:ab_glyph", "dep:image", "dep:ureq", "dep:base64", "dep:resvg", "dep:usvg", "dep:url"]
 
 [dependencies]
 taffy = { workspace = true }
@@ -20,6 +20,17 @@ ab_glyph = { workspace = true, optional = true }
 image = { version = "0.24", default-features = false, features = ["jpeg", "png", "webp"], optional = true }
 ureq = { version = "2.9", default-features = false, features = ["tls"], optional = true }
 base64 = { workspace = true, optional = true }
+# SVG rasterization for <img src="*.svg">, inline <svg>, and background-image
+# SVGs, all extremely common for logos/icons. Pure Rust, CPU, no system deps,
+# same deterministic-rendering guarantee as the rest of this crate.
+resvg = { version = "0.47", default-features = false, features = ["text", "raster-images"], optional = true }
+usvg = { version = "0.47", default-features = false, optional = true }
+url = { workspace = true, optional = true }
 cssparser = "0.34"
 [lib]
 path = "src/lib.rs"
+
+[[bin]]
+name = "paint_file"
+path = "src/bin/paint_file.rs"
+required-features = ["paint"]