New optional crate obscura-render, the scoped render layer for Obscura.
Phase 1 adds the layout core: a LayoutNode tree plus a viewport is laid
out with taffy (pure Rust) and border-box geometry is returned per node,
mirroring the input tree.
The crate is a workspace member but not yet a dependency of any other
crate, so the default scraping build is unchanged. Geometry-from-DOM,
JS integration, text, paint, and screenshot capture land in later
phases.
Unit tests cover block stacking, flex row layout, and padding.
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