refactor(builder): remove dead per-page headCode field (D9)

PageData.headCode was never populated from anything but '' and never
read anywhere -- the real, live concept is SiteDesign.headCode (set
via the Code modal, wired into Preview/export in D8). Remove the dead
field from the PageData interface and its ~7 hardcoded headCode: ''
seeds/discards in PageContext.tsx (DEFAULT_PAGE, DEFAULT_HEADER,
DEFAULT_FOOTER, the context default value, addPage, setPagesCraftState,
replaceAllPages). Update the PageData literals in
useWhpApi.save.test.ts to match the trimmed interface.

Verified via grep that nothing reads page.headCode, and `npm run
build` (strict tsc) is clean -- no live reader broke.

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

craft/src/state/PageContext.tsx

@@ -116,8 +116,8 @@ export const DEFAULT_HEADER_STATE = JSON.stringify({
 
 const PageContext = createContext<PageContextValue>({
   pages: [],
-  headerPage: { id: HEADER_ID, name: 'Header', slug: '__header__', craftState: null, headCode: '' },
-  footerPage: { id: FOOTER_ID, name: 'Footer', slug: '__footer__', craftState: null, headCode: '' },
+  headerPage: { id: HEADER_ID, name: 'Header', slug: '__header__', craftState: null },
+  footerPage: { id: FOOTER_ID, name: 'Footer', slug: '__footer__', craftState: null },
   activePageId: 'home',
   isEditingHeader: false,
   isEditingFooter: false,
@@ -166,7 +166,6 @@ const DEFAULT_PAGE: PageData = {
   name: 'Home',
   slug: 'index',
   craftState: null,
-  headCode: '',
 };
 
 const DEFAULT_HEADER: PageData = {
@@ -174,7 +173,6 @@ const DEFAULT_HEADER: PageData = {
   name: 'Header',
   slug: '__header__',
   craftState: DEFAULT_HEADER_STATE,
-  headCode: '',
 };
 
 const DEFAULT_FOOTER: PageData = {
@@ -182,7 +180,6 @@ const DEFAULT_FOOTER: PageData = {
   name: 'Footer',
   slug: '__footer__',
   craftState: null,
-  headCode: '',
 };
 
 export const PageProvider: React.FC<{ children: ReactNode }> = ({ children }) => {
@@ -307,7 +304,6 @@ export const PageProvider: React.FC<{ children: ReactNode }> = ({ children }) =>
           name,
           slug: uniqueSlug(requestedSlug, prev.map((p) => p.slug)),
           craftState: null,
-          headCode: '',
         },
       ]);
 
@@ -380,7 +376,6 @@ export const PageProvider: React.FC<{ children: ReactNode }> = ({ children }) =>
       // ALWAYS the landing page → slug 'index' → file index.html.
       slug: i === 0 ? 'index' : p.slug,
       craftState: p.craftState,
-      headCode: '',
     })));
   }, []);
 
@@ -500,7 +495,6 @@ export const PageProvider: React.FC<{ children: ReactNode }> = ({ children }) =>
         name: p.name,
         slug,
         craftState: treeToState(p.tree),
-        headCode: '',
       };
     });
     setPages(built);