Framework adapters accept site option
Update all five framework adapter packages — @refrakt-md/sveltekit, @refrakt-md/astro, @refrakt-md/nuxt, @refrakt-md/next, @refrakt-md/eleventy — to accept a site?: string option. Each adapter resolves its target site via the shared normalizer from WORK-159, then reads contentDir, theme, target, packages, routeRules, icons, backgrounds from the resolved site entry instead of the top-level config. Single-site projects and the legacy flat shape continue to work without changes; multi-site repos pick a target per adapter.