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/src/lib.rs

@@ -16,6 +16,9 @@ use taffy::prelude::*;
 mod style;
 pub use style::compute_style;
 
+mod dom;
+pub use dom::{layout_dom, DomLayout};
+
 /// An axis-aligned rectangle in CSS pixels, relative to the containing block.
 #[derive(Debug, Clone, Copy, Default, PartialEq)]
 pub struct Rect {
@@ -125,7 +128,7 @@ fn read_node(tree: &TaffyTree, id: NodeId) -> NodeRect {
     }
 }
 
-fn to_taffy_style(style: &LayoutStyle) -> Style {
+pub(crate) fn to_taffy_style(style: &LayoutStyle) -> Style {
     let mut s = Style::DEFAULT;
     s.display = match style.display {
         Display::Block => taffy::style::Display::Block,