feat(website): set production site URL (relplay.org); record Phase B decisions

Set Starlight `site` to https://relplay.org (apex) — enables the
sitemap, canonical URLs, and Open Graph URL resolution; clears the
sitemap build warning. Record the resolved STYLE.md decisions:
single-version launch, fixed-dark cast theme (casts bake RGB colours,
so light/dark would need dual-theme recordings), and the site origin.

website/astro.config.mjs

@@ -10,8 +10,12 @@ import rdbmsLang, { rdbmsSyntax } from './src/grammars/rdbms.mjs';
 
 // https://astro.build/config
 export default defineConfig({
-	// TODO(Phase B/SEO): set `site` to the production URL once the domain is
-	// known — enables the sitemap and canonical/OG URLs.
+	// Production origin (apex domain). Enables the sitemap, canonical URLs, and
+	// Open Graph `og:url`/`og:image` resolution. A `www` host, if used, should
+	// 301-redirect here so a single canonical origin is advertised. Local dev is
+	// unaffected — this only changes *generated absolute* URLs, not where the
+	// dev/preview server binds.
+	site: 'https://relplay.org',
 	// Bind the dev/preview server to IPv4 loopback. Astro/Vite's default
 	// `localhost` bind resolves to IPv6 `::1` here, which breaks SSH
 	// `-L 4321:127.0.0.1:4321` tunnels (they target IPv4). Pinning 127.0.0.1