Rux styling, milestone M2.
Parses the <style> CSS with lightningcss (literal CSS, per Law 4), matches
rules against the template tree with our own small selector engine, applies
the cascade, and produces a styled rux_layout::Node tree. This is Stage 2
of docs/04-architecture.md, narrowed to the honored subset.
Selector support: tag, .class, #id, [role="…"], compound
(view.card), and all four combinators, descendant (.a .b), child
(.a > .b), next-sibling (.a + .b) and subsequent-sibling (.a ~ .b).
Specificity and source order resolve conflicts, as in CSS.