fix(builder): dedupe page slugs on add/rename/replaceAll (D3)

Two pages that slugify to the same string (e.g. both named "About")
previously both published to about.html, silently overwriting each
other on publish. Add a pure uniqueSlug(base, existingSlugs) helper
that appends -2, -3, ... on collision, and apply it in addPage,
renamePage, and replaceAllPages. The landing page's slug stays locked
to 'index' regardless of collisions, matching existing behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/state/PageContext.tsx

@@ -148,6 +148,19 @@ function slugify(name: string): string {
     .replace(/-+/g, '-');
 }
 
+/**
+ * Append `-2`, `-3`, … to `base` until it no longer collides with
+ * `existingSlugs`. Two pages that slugify to the same string (e.g. both
+ * named "About") must not both publish to `about.html` — the second write
+ * would silently overwrite the first on publish.
+ */
+export function uniqueSlug(base: string, existingSlugs: string[]): string {
+  if (!existingSlugs.includes(base)) return base;
+  let i = 2;
+  while (existingSlugs.includes(`${base}-${i}`)) i++;
+  return `${base}-${i}`;
+}
+
 const DEFAULT_PAGE: PageData = {
   id: 'home',
   name: 'Home',
@@ -278,7 +291,7 @@ export const PageProvider: React.FC<{ children: ReactNode }> = ({ children }) =>
 
   const addPage = useCallback(
     (name: string, slug: string) => {
-      const finalSlug = slug || slugify(name);
+      const requestedSlug = slug || slugify(name);
       const id = `page_${Date.now()}`;
 
       // Save current page first
@@ -289,7 +302,7 @@ export const PageProvider: React.FC<{ children: ReactNode }> = ({ children }) =>
         {
           id,
           name,
-          slug: finalSlug,
+          slug: uniqueSlug(requestedSlug, prev.map((p) => p.slug)),
           craftState: null,
           headCode: '',
         },
@@ -335,8 +348,10 @@ export const PageProvider: React.FC<{ children: ReactNode }> = ({ children }) =>
         // First page is the landing page — its slug is locked to 'index' so
         // the file always publishes to index.html regardless of the user-set
         // name. The display name can change freely.
-        const nextSlug = i === 0 ? 'index' : (slug || slugify(name));
-        return { ...p, name, slug: nextSlug };
+        if (i === 0) return { ...p, name, slug: 'index' };
+        const requestedSlug = slug || slugify(name);
+        const otherSlugs = prev.filter((pp) => pp.id !== pageId).map((pp) => pp.slug);
+        return { ...p, name, slug: uniqueSlug(requestedSlug, otherSlugs) };
       }),
     );
   }, []);
@@ -467,16 +482,23 @@ export const PageProvider: React.FC<{ children: ReactNode }> = ({ children }) =>
    */
   const replaceAllPages = useCallback((newPages: { name: string; tree: SerializedTreeNode }[]) => {
     if (newPages.length === 0) return;
-    const built = newPages.map((p, i) => ({
-      id: i === 0 ? 'home' : `page_${Date.now()}_${i}`,
-      name: p.name,
+    // Track slugs as they're assigned so later pages dedupe against earlier
+    // ones in the same batch (e.g. the AI generating two "About" pages).
+    const seenSlugs: string[] = [];
+    const built = newPages.map((p, i) => {
       // First page must publish to index.html so it serves at the site root.
       // Apache resolves '/' to index.html, not home.html — without this, the
       // AI's "Home" page lands at /home.html and visitors hit a blank root.
-      slug: i === 0 ? 'index' : slugify(p.name),
-      craftState: treeToState(p.tree),
-      headCode: '',
-    }));
+      const slug = i === 0 ? 'index' : uniqueSlug(slugify(p.name), seenSlugs);
+      seenSlugs.push(slug);
+      return {
+        id: i === 0 ? 'home' : `page_${Date.now()}_${i}`,
+        name: p.name,
+        slug,
+        craftState: treeToState(p.tree),
+        headCode: '',
+      };
+    });
     setPages(built);
     // Load the first page into the live canvas
     const firstState = built[0].craftState;