Host diagram CSS variables

Context Rails and role-lens host SVGs kept light Mermaid node fills after the Themed SVG runtime could apply the ftn-diagram host namespace. Inline style="fill:#… !important" beat the CSS variables.

Diagram generation now includes stylesheet bindings and pins local @dev-centr/themed-svg 0.2.2 so those inline paints become var(--themed-svg-ftn-diagram-…) with light fallbacks. Dark mode recolors nodes once the runtime upgrades the marked image.