Date sitemap lastmod from the build pipeline; warn on skipped pages

- bunext build records when each page's client bundle hash last changed
  in .bunext/page-lastmod.json (sitemap.lastmod_file to relocate). The
  hash covers the page's whole import graph, so component edits re-date
  every page that uses them. The sitemap uses these dates as lastmod
- Add sitemap.default_lastmod
- BREAKING: use_file_lastmod now defaults to false (file mtimes are reset
  by checkouts); it is a fallback after the build record
- Warn when a page or __root.tsx is skipped because its default export
  is not at the start of a line (e.g. export { Page as default })
- Replace leaking module mocks in tests (log, grab-root-file-path) with
  spies and fixtures

Co-Authored-By: Claude Opus 5.5 <[email protected]>

dist/utils/grab-dir-names.d.ts

@@ -29,5 +29,6 @@ export type DirNames = {
     BUNDLER_CTX_MAP_JSON: string;
     SSR_BUNDLER_CTX_MAP_JSON: string;
     BUILD_INFO_JSON: string;
+    PAGE_LASTMOD_JSON: string;
 };
 export default function grabDirNames(): DirNames;