feat: course website (Astro Starlight)

Pages (index, disclaimer, about), lockfile, root test:site script, fix for
Starlight 0.42 sidebar config and <details> rendering in generated MDX.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

site/scripts/sync-content.mjs

@@ -131,6 +131,10 @@ export function mdxSafeProse(prose) {
       let s = p.s.replace(/<!--[\s\S]*?-->/g, "");
       s = s.replace(/[{}]/g, (c) => `\\${c}`);
       s = s.replace(/<(?!\/?(?:details|summary)\s*>)/g, "&lt;");
+      // Put each details/summary tag on its own line with blank lines around it, so the
+      // markdown inside <details> is parsed as markdown rather than as JSX text.
+      s = s.replace(/[ \t]*(<details>|<\/details>|<summary>[^\n]*?<\/summary>)[ \t]*/g, "\n\n$1\n\n");
+      s = s.replace(/\n{3,}/g, "\n\n");
       return s;
     })
     .join("");

site/scripts/sync-content.test.mjs

@@ -79,7 +79,7 @@ test("sync_pairs_code_tabs", () => {
 test("sync_strips_comments_and_escapes_mdx", () => {
   const out = mdxSafeProse("a <!-- Task 17: re-verify --> b {x} and `{kept}` <details><summary>S</summary> 1 < 2");
   assert.doesNotMatch(out, /<!--/);
-  assert.match(out, /a  b \\\{x\\\} and `\{kept\}` <details><summary>S<\/summary> 1 &lt; 2/);
+  assert.match(out, /a  b \\\{x\\\} and `\{kept\}`\n\n<details>\n\n<summary>S<\/summary>\n\n1 &lt; 2/);
   const root = fixture({ "01-a": `# A\n\n<!-- hidden -->\n\n${FENCE}rust\nif a < b { <!-- keep --> }\n${FENCE}\n` });
   const page = run(root)("01-a.mdx");
   assert.doesNotMatch(page, /hidden/);