render: DOM to layout tree and per-node geometry

Walk a live DomTree, compute each element's style (tag UA defaults plus
inline style), build a taffy layout tree, and return border-box geometry
keyed by NodeId. Block/flex structure, explicit sizes, and the box model
are correct; text nodes do not yet contribute to size (inline/text
layout lands with the paint phase).

This completes the layout core: a real DOM now produces real geometry,
pure Rust, behind the opt-in render crate. JS exposure is next.

crates/obscura-render/Cargo.toml

@@ -8,6 +8,7 @@ description = "Optional scoped render layer for Obscura: layout (taffy) and, wit
 
 [dependencies]
 taffy = { workspace = true }
+obscura-dom = { workspace = true }
 
 [lib]
 path = "src/lib.rs"