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/STYLE.md

@@ -191,9 +191,12 @@ learners, but is valuable more broadly.) Concretely:
 - **Embedding.** `Demo.astro` (the WASM-swap seam, ADR-website-001 §3) wraps
   `Cast.astro` (asciinema-player island). Call sites use `Demo`.
 - **Conventions.** Terminal geometry per cast (default 90×26); file name =
-  cast `name`; player theme `asciinema` for now. **[OPEN]**: light/dark player
-  theme sync with the Starlight theme toggle (player theme is currently fixed;
-  refine in Phase B).
+  cast `name`; player theme `asciinema`. **Cast theme — fixed dark for launch
+  (DECIDED 2026-06-11).** Casts bake the app's dark theme into the recording as
+  RGB SGR codes, so the player theme can't recolour them; true light/dark would
+  require recording light-theme cast variants and swapping via the `Demo` seam.
+  Deferred as not worth the doubled asset/recording cost for launch; revisit
+  with the eventual full re-record.
 
 ## Formatting  [DECIDED, refine]
 
@@ -229,9 +232,16 @@ significant).
 6. ~~Reference generation vs hand-writing~~ → **hand-write now** (command
    surface is settled bar H1a output; small later adjustments expected).
 
+**Resolved (2026-06-11):**
+7. ~~Versioning~~ → **single-version for launch.** No version selector; one can
+   be added at/after v1 if the docs ever need to diverge by version.
+   Cast light/dark → **fixed dark for launch** (see *asciinema casts* above).
+   `site` set to **`https://relplay.org`** (apex) — enables sitemap, canonical,
+   and Open Graph URL resolution; a `www` host, if used, 301-redirects to apex.
+
 **Still open:**
-7. **Versioning.** Version selectors at/after v1, or single-version for
-   launch? (Leaning single-version for launch.)
-8. **SEO/meta conventions.** Title/description patterns, Open Graph — settle
-   with Phase B (landing) and the `site` URL. Also: light/dark player-theme
-   sync for embedded casts.
+8. **SEO/meta — remaining.** Per-page titles/descriptions are set; canonical +
+   basic Open Graph (`og:title`/`og:url`) now auto-generate from `site`. What
+   remains: **Open Graph social-card images** (`og:image`) — needs a card
+   design, so it travels with the branding work (logo + palette, TBD with the
+   user; the existing favicon is *not* a brand guideline).

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