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/paint.rs

@@ -9,7 +9,7 @@ use obscura_dom::tree::DomTree;
 use tiny_skia::{Color, FillRule, GradientStop, LinearGradient, Paint, PathBuilder, Pixmap, Point, Rect, SpreadMode, Transform};
 use ab_glyph::{Font, FontRef, PxScale, ScaleFont};
 
-static FONT_BYTES: &[u8] = include_bytes!("../assets/dejavu-sans.ttf");
+static FONT_BYTES: &[u8] = include_bytes!("../assets/liberation-sans.ttf");
 
 use crate::layout_dom_with_images;