render: paint backgrounds to a tiny-skia pixmap

Adds a paint feature to obscura-render (tiny-skia: pure Rust, CPU,
deterministic, no system deps). paint_dom lays out the DOM and fills
each element's border box with its parsed background-color over a white
page, in tree order so descendants paint over ancestors.

Style parsing now also extracts background-color (hex #rgb/#rgba/
later enhancements.

crates/obscura-render/Cargo.toml

@@ -6,9 +6,16 @@ license.workspace = true
 repository.workspace = true
 description = "Optional scoped render layer for Obscura: layout (taffy) and, with the paint feature, text and raster output for screenshots."
 
+[features]
+default = []
+# Rasterize the laid-out tree to a pixmap (tiny-skia). Off by default; the
+# layout core (getBoundingClientRect etc.) works without it.
+paint = ["dep:tiny-skia"]
+
 [dependencies]
 taffy = { workspace = true }
 obscura-dom = { workspace = true }
+tiny-skia = { workspace = true, optional = true }
 
 [lib]
 path = "src/lib.rs"