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/page-lastmod-record.d.ts

@@ -0,0 +1,19 @@
+import type { BundlerCTXMap, BunextPageLastmodRecord } from "../types";
+/**
+ * # Path of the page last-modified record
+ */
+export declare function grabPageLastmodFile(lastmod_file?: string): string;
+export declare function readPageLastmodRecord(lastmod_file?: string): BunextPageLastmodRecord | undefined;
+/**
+ * # Record when each page's client bundle last changed
+ * A page's bundle hash covers its whole client import graph (including
+ * the root layout), so a component edit re-dates every page that uses it.
+ * Unchanged pages keep their previous date; removed pages are dropped.
+ */
+export declare function updatePageLastmodRecord({ ctx_map, build_date, lastmod_file, }: {
+    ctx_map: {
+        [local_path: string]: BundlerCTXMap;
+    };
+    build_date?: Date;
+    lastmod_file?: string;
+}): BunextPageLastmodRecord;