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/types/index.d.ts

@@ -438,10 +438,21 @@ export type BunextGenerateSitemapParams = {
      */
     additional_paths?: BunextSitemapAdditionalPaths | (() => BunextSitemapAdditionalPaths | Promise<BunextSitemapAdditionalPaths>);
     /**
-     * Use each page file's modification time as `lastmod`. Defaults to
-     * `true`.
+     * Fall back to each page file's modification time as `lastmod` when
+     * the build record has no date for it. Defaults to `false`: checkouts
+     * reset modification times, so they are rarely meaningful.
      */
     use_file_lastmod?: boolean;
+    /**
+     * `lastmod` for entries that have no other date
+     */
+    default_lastmod?: Date | string;
+    /**
+     * Page last-modified record written by `bunext build`, relative to the
+     * project root. Defaults to `.bunext/page-lastmod.json`. Point it at a
+     * cached or committed location when builds start from a fresh checkout.
+     */
+    lastmod_file?: string;
     /**
      * Append a trailing slash to every non-root path. Defaults to `false`.
      */
@@ -508,3 +519,15 @@ export type BunextRobotsConfig = BunextGenerateRobotsTxtParams & {
      */
     output?: string;
 };
+/**
+ * Written by `bunext build`. Keyed by page path relative to the project
+ * root. `lastmod` is the build date at which `fingerprint` last changed.
+ */
+export type BunextPageLastmodRecord = {
+    pages: {
+        [local_path: string]: {
+            fingerprint: string;
+            lastmod: string;
+        };
+    };
+};