render: scaffold obscura-render crate with taffy layout core

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.

crates/obscura-render/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "obscura-render"
+version.workspace = true
+edition.workspace = true
+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."
+
+[dependencies]
+taffy = { workspace = true }
+
+[lib]
+path = "src/lib.rs"