v0.1.0 of layout_engine

Framework-agnostic layout engine extracted from artisanal. Provides BoxConstraints, RenderObject, RenderBox, RenderRow/RenderColumn, and RenderPadding — no dependencies on any rendering backend. Changelog excerpt: - Initial public release of the framework-agnostic layout engine. - Added constraint-based geometry, flex, padding, wrap, stack, scroll, and element-tree primitives.