Refresh every stale post route during validation.

specs/generation.allium

@@ -296,8 +296,9 @@ rule ValidateSite {
     --   missing: expected routes with no non-empty index.html
     --   extra: index.html routes absent from the expected set, including
     --          zero-byte files
-    --   stale: existing post routes whose source mtime is more than one second
-    --          newer than both the output mtime and tracked generation time
+    --   stale: every existing generated route for a post whose source mtime is
+    --          more than one second newer than both the output mtime and
+    --          tracked generation time, including canonical and flat page URLs
     -- Standalone HTML, XML, JSON, assets, Pagefind files, and content hashes are
     -- not validation comparison inputs.
     ensures: ValidationReport(missing_pages, extra_pages, stale_pages)