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/commands/sitemap/index.js

@@ -6,6 +6,7 @@ import grabDirNames from "../../utils/grab-dir-names";
 import grabConfig from "../../functions/grab-config";
 import grabConstants from "../../utils/grab-constants";
 import { generateSitemapFiles } from "../../utils/generate-sitemap";
+import { grabPageLastmodFile } from "../../utils/page-lastmod-record";
 import generateRobotsTxt, { RobotsTxtGeneratedMarker, } from "../../utils/generate-robots-txt";
 export default function () {
     return new Command("sitemap")
@@ -18,6 +19,10 @@ export default function () {
         global.BUNEXT_CONFIG = (await grabConfig()) || {};
         global.BUNEXT_CONSTANTS = grabConstants();
         const { output, chunk_size, chunks_dir, ...sitemap_config } = global.BUNEXT_CONFIG.sitemap || {};
+        if (!existsSync(grabPageLastmodFile(sitemap_config.lastmod_file))) {
+            log.warn(`No page lastmod record found. Run \`bunext build\` first ` +
+                `so pages get lastmod dates.`);
+        }
         let sitemap_file;
         try {
             const output_path = options.output || output;