feat(site): phase 1 — the foundation

Astro 7 with the Node adapter, Starlight mounted at /docs, the token file, both
self-hosted typefaces, the layout shell, and the two build-time checks from §12.

The palette's gold and cyan are sampled from runic-emblem.png rather than
guessed, per §11: 494,059 opaque pixels binned by hue, each value annotated with
its measured contrast against the ground, and restricted rather than brightened
where a ratio fails.

- checkTokens.mjs fails the build on any colour literal outside tokens.css,
  which is what keeps §7's "recolouring is a file copy" promise true.
- checkFacts.mjs re-reads all 14 externally-sourced facts from their authorities
  over the Gitea API and fails on disagreement. It also enforces D13: no email
  address in the source outside brand-default/brand.json.
- Both were negative-tested; neither has ever been allowed to pass by default.

§6 asks for output:'server' with per-page prerender=true. Astro 7 expresses the
same runtime shape as output:'static' with an adapter, opting individual routes
out — so the default is static rather than accidentally server-rendered.

Co-Authored-By: Claude <noreply@anthropic.com>

src/pages/404.astro

@@ -0,0 +1,32 @@
+---
+import Base from '../layouts/Base.astro';
+
+/**
+ * Ours rather than Starlight's (`disable404Route: true` in astro.config.mjs). A reader who
+ * mistypes a marketing URL should not land in documentation chrome with a sidebar of pages
+ * they were not looking for.
+ */
+---
+
+<Base title="Page not found" description="That page does not exist on this site.">
+  <section class="page notfound">
+    <p class="eyebrow">404</p>
+    <h1>That page does not exist</h1>
+    <p class="prose">
+      The link may be out of date, or the page may not be written yet — this site is being
+      built in phases.
+    </p>
+    <p><a href="/">Back to the homepage</a> &middot; <a href="/docs/">Documentation</a></p>
+  </section>
+</Base>
+
+<style>
+  .notfound {
+    padding-block: clamp(3rem, 9vw, 6rem);
+  }
+
+  .notfound h1 {
+    margin: 0 0 1rem;
+    color: var(--gold);
+  }
+</style>