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.js

@@ -33,6 +33,7 @@ export default function grabDirNames() {
     const BUNDLER_CTX_MAP_JSON = path.join(BUNX_CWD_DIR, "bundler-ctx-map.json");
     const SSR_BUNDLER_CTX_MAP_JSON = path.join(BUNX_CWD_DIR, "ssr-bundler-ctx-map.json");
     const BUILD_INFO_JSON = path.join(BUNX_CWD_DIR, "build-info.json");
+    const PAGE_LASTMOD_JSON = path.join(BUNX_CWD_DIR, "page-lastmod.json");
     return {
         ROOT_DIR,
         SRC_DIR,
@@ -64,6 +65,7 @@ export default function grabDirNames() {
         BUNDLER_CTX_MAP_JSON,
         SSR_BUNDLER_CTX_MAP_JSON,
         BUILD_INFO_JSON,
+        PAGE_LASTMOD_JSON,
     };
 }
 // const NODE_MODULES_DIR = path.resolve(