feat(cli): preserve custom sections after solution during docs sync

apps/cli/sync.ts

@@ -359,6 +359,24 @@ function ensureLabel(page: string, h: Header): string {
   return page.replace(/^sidebar:$/m, `sidebar:\n${label}`);
 }
 
+/**
+ * Offset just past the script-owned `## Solution` section: the next top-level
+ * heading that is not inside a code fence, or end of file. Sections a human
+ * appends after the solution (`## Explanation`, …) survive regeneration.
+ */
+function solutionEnd(page: string, from: number): number {
+  const lines = page.slice(from).split("\n");
+  let offset = from + lines[0]!.length + 1; // past the '## Solution' line itself
+  let fenced = false;
+  for (let i = 1; i < lines.length; i++) {
+    const line = lines[i]!;
+    if (line.startsWith("```")) fenced = !fenced;
+    else if (!fenced && line.startsWith("## ")) return offset;
+    offset += line.length + 1;
+  }
+  return page.length;
+}
+
 // Existing docs pages, keyed by leetcode number from the frontmatter title.
 const pages = new Map<number, string>(); // num -> path relative to DOCS
 for (const rel of (await Array.fromAsync(new Bun.Glob("**/*.mdx").scan(DOCS))).sort()) {
@@ -386,7 +404,9 @@ for (const sol of ordered) {
       report.push([workRef, existing, "ERROR: no '## Solution' heading"]);
       continue;
     }
-    const next = ensureLabel(page.slice(0, at) + solutionSection(sol), sol.header);
+    const tail = page.slice(solutionEnd(page, at));
+    const body = page.slice(0, at) + solutionSection(sol) + (tail ? `\n${tail}` : "");
+    const next = ensureLabel(body, sol.header);
     const actions: string[] = [];
     if (existing !== name) {
       await rename(join(DOCS, existing), join(DOCS, name));