render: match Chrome font metrics with Liberation Sans/Serif/Mono

Chrome's default sans-serif on this class of host is Liberation Sans, not
DejaVu (measured: sans-serif == Arial == Helvetica == Liberation width,
DejaVu ~12% wider). Forcing DejaVu drifted all text wider and shifted
every line. Bundle Liberation Sans/Serif/Mono (DejaVu kept as glyph
fallback), default to Liberation Sans, and resolve font-family to the
right generic (monospace/code -> Mono, serif -> Serif, else -> Sans).
A controlled text page drops from ~15-20% to 3.5% pixel mismatch and
every line lands within 1px of Chrome.

crates/obscura-render/src/lib.rs

@@ -239,6 +239,10 @@ pub struct LayoutStyle {
     /// root font-sizes. `None` when font-size was absolute or unset.
     pub font_size_raw: Option<Dimension>,
     pub font_weight: Option<String>,
+    /// The computed `font-family` list, lowercased. Inherited. The text engine
+    /// resolves it to a bundled face (Liberation Sans/Serif/Mono) the way
+    /// Chromium picks a generic family on this host.
+    pub font_family: Option<String>,
     pub align_items: Option<taffy::AlignItems>,
     pub flex_direction: Option<taffy::FlexDirection>,
     pub flex_wrap: Option<taffy::FlexWrap>,