Fix icon rendering, calc()/var() length resolution, and approximate CSS float

Icons were rendering as either overlapping solid squares or not at all,
  text ran under the infobox instead of beside it, and general.css parsing
  had two more silent-corruption bugs on top of the ones fixed earlier:

  - A CSS declaration list was split on every literal semicolon, so any
    value containing one inside a quoted string or url() (a data: URI
    with a MIME parameter, e.g. url(data:image/svg+xml;utf8,<svg...>),
    extremely common for small inline icon SVGs) got torn into two
    malformed halves and silently dropped. This is why mask-image never
    applied to the TOC toggle icons at all.
  - data: URIs were only decoded as base64. Inline SVGs are just as
    commonly written as data:image/svg+xml;utf8,<percent-escaped text>,
    which needs percent-decoding instead; added that path.
  - top/left/right/bottom/width/height only handled bare lengths and a
    flat calc(a +/- b). Real values nest var(--x, fallback), calc(), and
    min()/max() inside each other (Wikipedia's icon offset math is
    literally calc(-1 * 22px / 2), and its icon sizing is
    calc(max(calc(var(--font-size-medium,1rem) + 4px),10px))). Replaced
    the flat evaluator with a small recursive resolver that handles all
    three, nested arbitrarily. This is what was placing the TOC toggle
    button's icon on top of the section text instead of in the left
    margin where it belongs.

  Added actual feature support once the parsing was correct:

  - mask-image/-webkit-mask-image: the pattern behind most scalable
    colored icons on the modern web (an SVG or image used purely as an
    alpha stencil, tinted by background-color/color). Previously every
    masked icon rendered as an unmasked solid-color square.
  - float: left/right, approximated without a full per-line reflow
    engine (which taffy's block/flex/grid modes don't provide): a
    floated element is grouped into a row alongside the flow siblings
    that follow it, up to whichever comes first of the next heading or
    another float, then everything from there on reverts to normal
    full-width flow. This isn't general CSS float, but it directly
    targets the overwhelmingly common real shape (an infobox or image
    floated near the top of an article, with the rest of the content
    starting at the next section heading), and is what makes article
    text actually wrap beside an infobox instead of running underneath
    it.

  Refactored paint.rs's image fetch/decode/cache logic into shared
  helpers (fetch_bytes, raster_to_pixmap) reused by plain <img> painting,
  background-image, and the new mask-image path.

crates/obscura-render/src/lib.rs

@@ -113,6 +113,11 @@ pub struct LayoutStyle {
     /// `background_size`: without a known image size there is no leftover
     /// box space to position within.
     pub background_position: (f32, f32),
+    /// `mask-image`/`-webkit-mask-image: url(...)`: the ubiquitous "colored,
+    /// scalable icon" pattern (an SVG shape used as a stencil, tinted by
+    /// `background-color`/`color` instead of carrying its own colors). Without
+    /// this, every such icon paints as a solid filled square.
+    pub mask_image: Option<String>,
     /// Foreground (text) color for the paint step.
     pub color: Option<[u8; 4]>,
     pub border_color: Option<[u8; 4]>,
@@ -147,6 +152,17 @@ pub struct LayoutStyle {
     /// clipped box used for skip-links and screen-reader-only labels) actually
     /// invisible instead of painting its text wherever it lands.
     pub overflow_hidden: bool,
+
+    /// `float: left|right`. True CSS float needs per-line reflow around the
+    /// float's shape, which taffy's block/flex/grid modes do not do; see
+    /// `dom::group_float_zone` for the bounded approximation this drives.
+    pub float: Option<Float>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Float {
+    Left,
+    Right,
 }
 
 /// A node in the input layout tree. `text` is carried for the paint phase; it