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]>

CLAUDE.md

@@ -62,6 +62,7 @@ API routes live in `src/pages/api/` and follow standard Bun `Request → Respons
 
 ### Bundler artifact tracking
 `BundlerCTXMap` (stored in `global.BUNDLER_CTX_MAP`) maps each page to its bundled output path, content hash, and entrypoint. In production this is serialized to `public/pages/map.json` and loaded at startup.
+`bunext build` also updates `.bunext/page-lastmod.json` (`src/utils/page-lastmod-record.ts`): per page, a fingerprint of the bundle hash + CSS path and the build date it last changed. The sitemap uses these dates as `lastmod`.
 
 ### Config
 Consumer projects define `bunext.config.ts` at their project root. The `BunextConfig` type fields: `distDir`, `assetsPrefix`, `origin`, `globalVars`, `port`, `development`.