chore: scaffold website (Astro 6 + Starlight + Tailwind v4)

Phase A of docs/plans/20260604-adr-0042-website.md. Scaffolds the site
under website/ from the Starlight template; adds Tailwind v4 (via
@tailwindcss/vite) bridged to Starlight with @astrojs/starlight-tailwind
(src/styles/global.css + customCss). Production build is green: static
output, Pagefind search index, sharp image optimization.

Template placeholders (title, example pages, sidebar) are left for
Phase B/D. Reconciles the ADR/plan/index wording from "Astro 5" to
"Astro 6" to match the scaffolded toolchain.

docs/adr/0042-public-website-and-documentation-site.md

@@ -26,7 +26,7 @@ planning + `/runda` pass with the user and are recorded below.
 
 ## Decision
 
-1. **Stack — Astro 5 + Starlight + Tailwind v4.** Astro's content-first,
+1. **Stack — Astro 6 + Starlight + Tailwind v4.** Astro's content-first,
    zero-JS-by-default model with the Starlight docs theme fits a
    marketing-landing-plus-heavy-docs site better than the alternative
    considered, SvelteKit + Tailwind (the usual go-to here). Interactive
@@ -65,7 +65,7 @@ planning + `/runda` pass with the user and are recorded below.
      and the rusqlite **backup-API** undo (ADR-0006) → a SQL dump/restore.
    When taken up, this becomes its own ADR + iteration plan.
 
-4. **Hosting — portable static build; Vercel is the likely target.** Astro 5
+4. **Hosting — portable static build; Vercel is the likely target.** Astro 6
    builds to static HTML/CSS with no adapter, so the output deploys equally
    to Vercel, Cloudflare Pages, Netlify, or GitHub Pages. We avoid coupling
    to any one host. **No CI yet** — the site is tested locally (`pnpm dev` /