render: computed-style lite (inline + UA defaults)

Parse inline style attributes into the layout-relevant subset of
LayoutStyle: display, width/height, margin, padding, and border width,
including box shorthands (1-4 values) and per-side longhands. !important
is stripped; auto and percentages leave the size unset for phase 1.

Layered on a small built-in UA default sheet. A compact hand-rolled
tokenizer handles inline attributes without a cssparser dependency; a
real cascade can harden it later.

crates/obscura-render/src/lib.rs

@@ -13,6 +13,9 @@
 
 use taffy::prelude::*;
 
+mod style;
+pub use style::compute_style;
+
 /// An axis-aligned rectangle in CSS pixels, relative to the containing block.
 #[derive(Debug, Clone, Copy, Default, PartialEq)]
 pub struct Rect {